# C9 Gene: Structure, Function, and Clinical Significance ## Key Takeaways - C9 is the terminal effector of the complement system, essential for forming the Membrane Attack Complex (MAC) by polymerizing on the C5b-8 complex to create a transmembrane pore that lyses target cells. - Deficiency in C9, often due to nonsense mutations like R95X, confers a significant susceptibility to recurrent, invasive infections by *Neisseria meningitidis* and *Neisseria gonorrhoeae*. - Rare variants in the C9 gene are associated with an increased risk of Age-Related Macular Degeneration (AMD), suggesting a role for complement-mediated damage in retinal pathology. - Hepatitis B virus (HBV) employs immune evasion strategies by suppressing C9 synthesis via limiting the transcription factor USF-1, thereby inhibiting MAC formation and promoting viral persistence. - While C5 inhibitors like eculizumab indirectly block C9 function by preventing MAC assembly, direct C9 inhibitors are under investigation as therapeutic targets for complement-mediated diseases such as PNH and AMD. --- ## Executive Summary & Key Metadata The complement component 9 (C9) gene encodes the ninth and final component of the complement system, a serum glycoprotein essential for the formation of the Membrane Attack Complex (MAC). C9 is the terminal effector of the lytic pathway, polymerizing on the C5b-8 complex to create a transmembrane pore that disrupts the osmotic integrity of target cells, including Gram-negative bacteria, enveloped viruses, and aberrant host cells. Beyond its canonical role in innate immunity, C9 is increasingly recognized for its involvement in chronic inflammatory diseases, age-related macular degeneration (AMD), and host-pathogen immune evasion strategies. This manual provides a comprehensive, publication-grade analysis of the C9 gene, covering its genomic architecture, protein structure, signaling networks, pathogenic mutations, and clinical relevance. | **Attribute** | **Detail** | | :--- | :--- | | **HGNC Symbol** | C9 | | **UniProt Accession** | P02748 | | **Representative PDB ID** | True (e.g., 5FMW, 6D5L for MAC) | | **Chromosomal Locus** | 5p13.1 | | **Primary Molecular Function** | Terminal complement component; MAC pore formation; cytolysis | | **Disease & Pathology Associations** | C9 deficiency (Neisserial infections), Age-related Macular Degeneration (AMD), Cancer, HBV immune evasion | --- ## 1. Genomic Locus, Chromosomal Organization & Isoforms ### 1.1 Chromosomal Mapping and Locus Architecture The human C9 gene is located on the short arm of chromosome 5, specifically at cytogenetic band **5p13.1**. This localization was independently confirmed by two groups in 1989 using polymerase chain reaction (PCR) analysis of somatic cell hybrids and Southern blotting of genomic DNA [1][2]. The gene is situated within a region that is syntenic with the complement MAC gene cluster, which includes C6, C7, C8A, and C8B, although C9 is not physically clustered with these genes in the same contiguous block as C6 and C7 [3][4]. The precise genomic coordinates (GRCh38/hg38) span approximately 80 kilobases (kb) of genomic DNA, with the coding sequence distributed across 11 exons [5][6]. ### 1.2 Promoter Architecture and Transcriptional Regulation The 5' regulatory region of the C9 gene lacks a canonical TATA box but contains a high GC content and multiple potential transcription factor binding sites [7]. Witzel-Schlömp et al. (2001) structurally analyzed the 5' gene region and identified several regulatory elements critical for basal and inducible expression [7]. Key features include: - **GC Boxes**: Binding sites for the transcription factor Sp1, which is essential for the transcription of TATA-less genes. - **USF-1 Binding Sites**: The upstream stimulatory factor 1 (USF-1) has been identified as a critical transcription factor for C9 expression. Hepatitis B virus (HBV) has been shown to downregulate C9 synthesis by limiting the availability of USF-1, thereby inhibiting MAC formation and facilitating viral persistence [8]. - **Acute-Phase Response Elements**: The C9 gene is upregulated during inflammation. In amphibian models, LPS stimulation induces C9 expression in the liver, suggesting conserved acute-phase regulatory mechanisms [9]. The promoter likely contains binding sites for C/EBP and NF-κB, which are common mediators of acute-phase responses. ### 1.3 Exon-Intron Structure and Evolutionary Conservation The human C9 gene is composed of 11 exons, a structure that is largely conserved across vertebrates [5][6]. The intron-exon boundaries do not strictly correspond to the functional domains of the protein, suggesting that the gene evolved through exon shuffling and domain duplication events [6][3]. Comparative analysis of the C9 gene structure with other MAC components (C6, C7, C8A, C8B) reveals a shared evolutionary ancestry, with all genes encoding proteins that contain a common set of structural modules, including the MACPF domain [3][10]. The gene structure in fish species, such as the half-smooth tongue sole (*Cynoglossus semilaevis*), ayu (*Plecoglossus altivelis*), and grass carp (*Ctenopharyngodon idella*), shows a similar organization, with the full-length cDNA containing a 5' untranslated region (UTR), a single open reading frame (ORF), and a 3' UTR [11][12][13]. For instance, the ayu C9 cDNA is 2,125 nucleotides long, encoding a protein of approximately 65 kDa [12]. The evolutionary conservation of the C9 gene structure underscores its fundamental role in the innate immune system across jawed vertebrates [14]. ### 1.4 Alternative Splicing and Isoforms While C9 is primarily expressed as a single canonical transcript, alternative splicing events have been reported, particularly in the context of disease. In the C9orf72 gene (a distinct gene on chromosome 9, often abbreviated as "C9" in the context of ALS/FTD), aberrant splicing is a hallmark of the disease. However, for the complement C9 gene on chromosome 5, alternative splicing is less common. The primary transcript encodes a single-chain polypeptide of 559 amino acids, including a signal peptide of 21 amino acids that is cleaved upon secretion [5]. Some studies have suggested the existence of minor splice variants, but their functional significance remains to be fully characterized. --- ## 2. 3D Protein Domain Architecture & Structural Biology ### 2.1 Primary Structure and Domain Organization The C9 protein is a single-chain serum glycoprotein with a molecular weight of approximately 71 kDa (unmodified). It is a member of the Membrane Attack Complex/Perforin (MACPF) superfamily, which includes perforin, C6, C7, C8α, and C8β. The protein is organized into several distinct structural domains from the N-terminus to the C-terminus: 1. **Signal Peptide (aa 1-21)**: Directs the nascent polypeptide into the endoplasmic reticulum for secretion. 2. **Thrombospondin Type 1 (TSP1) Domain (aa 22-93)**: This domain is involved in protein-protein interactions and is found in many extracellular matrix proteins and complement components. In C9, it is thought to mediate interactions with other MAC components, particularly C5b-8. 3. **Low-Density Lipoprotein Receptor Class A (LDLRA) Domain (aa 94-133)**: This domain is characterized by a series of cysteine-rich repeats that bind calcium ions. It is involved in ligand binding and may contribute to the interaction of C9 with the C5b-8 complex. 4. **MACPF Domain (aa 134-390)**: This is the pore-forming domain, which is the defining feature of the MACPF superfamily. It consists of a central β-sheet core flanked by two α-helical bundles (CH1 and CH2). Upon activation, these helices undergo a dramatic conformational change, unfolding into amphipathic β-hairpins that insert into the target cell membrane to form the pore. 5. **Epidermal Growth Factor (EGF)-like Domain (aa 391-450)**: This calcium-binding domain is involved in protein-protein interactions and is thought to play a role in the regulation of C9 polymerization. 6. **C-Terminal Domain (CTD) (aa 451-559)**: This domain is unique to C9 and is not found in other MACPF proteins. It contains a conserved cysteine residue (Cys-559) that forms a disulfide bond with Cys-94 in the LDLRA domain, creating a large loop. The CTD is essential for the spontaneous polymerization of C9 into the tubular poly-C9 structure. ### 2.2 Tertiary and Quaternary Structure The three-dimensional structure of soluble C9 has been determined by X-ray crystallography, revealing a globular, elongated architecture. The protein is approximately 120 Å long and 50 Å wide. The MACPF domain forms the central core of the molecule, with the TSP1 and LDLRA domains extending from the N-terminus and the EGF and CTD domains extending from the C-terminus. The transition from the soluble form to the membrane-inserted pore involves a massive conformational rearrangement. The soluble C9 monomer is a "compact" form, where the MACPF domain is in a closed state. Upon binding to the C5b-8 complex, C9 undergoes a conformational change that exposes a membrane-binding interface. The two α-helical bundles (CH1 and CH2) of the MACPF domain unfold and refold into two amphipathic β-hairpins. These β-hairpins insert into the lipid bilayer, forming a β-barrel-like structure that spans the membrane. This insertion is the critical step in pore formation. ### 2.3 The Membrane Attack Complex (MAC) and Poly-C9 The MAC is a supramolecular complex that forms a lytic pore on target membranes. The assembly pathway is as follows: 1. C5b, C6, and C7 form the C5b-7 complex, which inserts into the target membrane. 2. C8 binds to the C5b-7 complex, forming the C5b-8 complex. 3. A single C9 molecule binds to the C5b-8 complex, initiating polymerization. 4. Additional C9 molecules (up to 12-18) are recruited and polymerize around the C5b-8 complex, forming the complete MAC. The resulting MAC is a hollow cylinder with an internal diameter of approximately 100 Å. The pore is formed by the β-hairpins from each C9 molecule, which come together to form a β-barrel. The structure of the fully assembled MAC has been resolved by cryo-electron microscopy (cryo-EM), confirming the architecture of the poly-C9 pore. The C9 protein can also polymerize spontaneously in the absence of the C5b-8 complex, forming a tubular structure known as poly-C9, which is functionally similar to the MAC but less efficient at lysing cells. ### 2.4 Interactive 3D Visualizer To explore the three-dimensional structure of the C9 protein in detail, including its domain architecture and the conformational changes associated with pore formation, please use the interactive visualizer below. [Interactive 3D Protein Visualizer: Load C9 (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=P02748) --- ## 3. Cellular Signaling Pathways & Molecular Function ### 3.1 The Complement Cascade and Terminal Pathway C9 is the terminal component of the complement system, a critical part of the innate immune system. The complement cascade can be activated through three distinct pathways: the classical, lectin, and alternative pathways. All three converge on the formation of the C5 convertase, which cleaves C5 into C5a and C5b. C5b initiates the terminal pathway, leading to the assembly of the MAC. The primary molecular function of C9 is to form the lytic pore of the MAC. This process is highly regulated to prevent non-specific damage to host cells. The regulation occurs at multiple levels: - **Fluid-Phase Regulators**: Proteins such as Clusterin and Vitronectin bind to the C5b-7 complex in the fluid phase, preventing its insertion into host cell membranes. - **Membrane-Bound Regulators**: Proteins such as CD59 (Protectin) and CD35 (CR1) inhibit the formation of the MAC on host cells. CD59 binds to the C8 and C9 components of the MAC, preventing the full polymerization of C9 and the formation of a lytic pore [15]. ### 3.2 C9 in Antimicrobial Defense The primary role of C9 is the lysis of Gram-negative bacteria. The MAC forms on the outer membrane of these bacteria, disrupting the permeability barrier and leading to cell death. The importance of C9 in host defense is underscored by the clinical phenotype of C9 deficiency, which is strongly associated with recurrent, invasive infections caused by *Neisseria meningitidis* and *Neisseria gonorrhoeae* [5][16][17]. In fish, C9 plays a similar role in innate immunity. Studies in rohu (*Labeo rohita*), pufferfish (*Takifugu rubripes*), and largemouth bronze gudgeon (*Coreius guichenoti*) have shown that C9 expression is upregulated in response to bacterial and parasitic infections [18][19][20]. For example, in the largemouth bronze gudgeon, C9 expression is significantly induced upon infection with *Ichthyophthirius multifiliis*, a ciliate parasite that causes white spot disease [18]. The functional role of C9 in fish is further supported by studies showing that recombinant C9 protein can influence the transcriptome of peripheral blood leukocytes, modulating immune-related gene expression [11]. ### 3.3 C9 and CD59: A Regulatory Axis The activity of C9 is tightly controlled by the membrane-bound complement regulatory protein CD59. CD59 binds to the C8 and C9 components of the assembling MAC, preventing the incorporation of additional C9 molecules and thus blocking the formation of a lytic pore. This interaction is critical for protecting host cells from complement-mediated damage. A study by Mu et al. (2023) in a teleost fish model demonstrated that C9 regulates complement-mediated cell lysis in association with CD59 to resist bacterial infection [15]. This study showed that the balance between C9 and CD59 is crucial for effective immune defense while preventing self-damage. The interaction between C9 and CD59 is a potential target for therapeutic intervention in diseases where complement-mediated damage is pathological, such as AMD and various autoimmune disorders. ### 3.4 Non-Lytic Functions of C9 Beyond its role in cell lysis, C9 has been implicated in several non-lytic functions. Sublytic concentrations of the MAC can activate various signaling pathways in host cells, leading to cell activation, proliferation, and inflammation. These effects are mediated by the influx of calcium and the activation of intracellular signaling cascades, including the MAPK and PI3K/AKT pathways. C9 has also been shown to play a role in the regulation of inflammation. In the context of age-related macular degeneration (AMD), C9 is a component of drusen, the extracellular deposits that accumulate beneath the retinal pigment epithelium (RPE). The presence of C9 in drusen suggests that local complement activation and MAC formation contribute to the chronic inflammation and RPE cell death that characterize AMD [21][22][23]. ### 3.5 Protein-Protein Interaction Networks The function of C9 is dependent on its interactions with other proteins. Key interaction partners include: - **C5b, C6, C7, C8**: These proteins form the C5b-8 complex, which is the platform for C9 binding and polymerization. - **CD59**: A negative regulator that binds to C9 and inhibits MAC formation. - **Clusterin and Vitronectin**: Fluid-phase inhibitors that bind to the C5b-7 complex and prevent MAC assembly. - **USF-1**: A transcription factor that regulates C9 gene expression [8]. These interactions are critical for the precise regulation of the terminal complement pathway. Dysregulation of any of these interactions can lead to either immunodeficiency (due to excessive inhibition) or autoimmune/inflammatory disease (due to insufficient inhibition). --- ## 4. Pathogenic Hotspot Mutations & Clinical Differentials ### 4.1 C9 Deficiency and Neisserial Infections Hereditary C9 deficiency is the most common complement deficiency in Japan, where it is found in approximately 0.1% of the population. It is inherited in an autosomal recessive manner and is strongly associated with increased susceptibility to recurrent, invasive infections with *Neisseria meningitidis* and *Neisseria gonorrhoeae* [5][16][17]. The most common cause of C9 deficiency in the Japanese population is a nonsense mutation in exon 4, which results in the substitution of arginine at position 95 with a stop codon (Arg95Stop, R95X) [16][17]. This mutation leads to the production of a truncated, non-functional protein. The R95X mutation is also found in other populations, and its frequency varies among different ethnic groups [24]. Witzel-Schlömp et al. (1997) identified two additional mutations causing C9 deficiency and revised the gene structure [5]. These mutations include a frameshift mutation and a splice-site mutation, both of which result in the complete loss of C9 protein function. The identification of these mutations has important implications for genetic counseling and the diagnosis of C9 deficiency. ### 4.2 C9 Mutations in Age-Related Macular Degeneration (AMD) Age-related macular degeneration (AMD) is a leading cause of blindness in the elderly. Chronic complement activation and inflammation are key drivers of AMD pathogenesis. Rare variants in complement genes, including C9, have been associated with an increased risk of advanced AMD [21][23]. Seddon et al. (2013) conducted a large-scale sequencing study and identified rare variants in CFI, C3, and C9 that are associated with a high risk of advanced AMD [23]. Functional analyses of these rare C9 variants have shown that some of them lead to altered complement activity, either by increasing MAC formation or by impairing the regulation of the complement pathway [21]. The C9-R95X polymorphism has also been investigated in patients with neovascular AMD, although its role in disease susceptibility remains unclear [1]. ### 4.3 C9 in Cancer The role of C9 in cancer is complex and context-dependent. On one hand, the MAC can lyse tumor cells, and C9 expression is often downregulated in tumors, allowing them to escape complement-mediated immune surveillance. On the other hand, sublytic MAC deposition can promote tumor cell proliferation, migration, and invasion. Studies have shown that C9 expression is altered in various types of cancer. For example, in gastrointestinal cancers, complement-related genes, including C9, are part of a hepatobiliary-specific immune gene signature [2]. The expression of C9 in the tumor microenvironment may influence the efficacy of immunotherapy. ### 4.4 Other Disease Associations - **HIV/HBV Coinfection**: Proteomic analysis has identified C9 as a differentially expressed protein in HIV/HBV coinfected patients, suggesting a role in the immune response to these viral infections [3]. - **Ebola Virus Disease**: Gene expression analysis has identified C9 as part of a hub gene network that distinguishes fatal from survivor outcomes in Ebola virus disease [4]. - **Spinal Cord Injury**: In a rat model of acute spinal cord injury, gene recombination of sCR1 (a complement inhibitor) was shown to affect the expression of C9 and Clusterin, suggesting a role for the terminal complement pathway in the secondary injury cascade [5]. ### 4.5 Clinical Differentials The clinical presentation of C9 deficiency is similar to that of other terminal complement component deficiencies (C5, C6, C7, C8). Patients typically present with recurrent meningococcal or gonococcal infections. The diagnosis is confirmed by measuring serum C9 levels and by genetic testing for mutations in the C9 gene. It is important to distinguish C9 deficiency from deficiencies of other terminal complement components, as the management and prophylactic strategies may differ. --- ## 5. Host-Pathogen & Viral Interactions ### 5.1 Hepatitis B Virus (HBV) Evasion Hepatitis B virus (HBV) has developed sophisticated mechanisms to evade the host immune system, including the complement system. A study by Baidya et al. (2022) demonstrated that HBV suppresses complement C9 synthesis by limiting the availability of the transcription factor USF-1 [8]. This suppression inhibits the formation of the MAC, allowing HBV-infected cells to escape complement-mediated lysis. This is a critical immune evasion strategy that contributes to the persistence of chronic HBV infection. The mechanism involves the HBV X protein (HBx), which interacts with USF-1 and sequesters it, preventing it from binding to the C9 promoter. This results in decreased C9 gene transcription and reduced C9 protein levels. The study also showed that restoring USF-1 expression or C9 levels could enhance MAC formation and promote the clearance of HBV-infected cells, suggesting a potential therapeutic strategy for chronic HBV infection. ### 5.2 Vaccinia Virus C9 Protein Vaccinia virus, the vaccine used to eradicate smallpox, encodes a protein called C9 that is an Ankyrin Repeat/F-Box protein [6]. This viral C9 protein is a newly identified antagonist of the type I interferon-induced antiviral state. It functions by targeting cellular proteins for degradation via the ubiquitin-proteasome pathway, thereby inhibiting the antiviral response. This is an example of a viral protein that has evolved to mimic or interfere with host cellular processes, although it is not directly related to the complement C9 protein. ### 5.3 Bacterial Interactions The MAC is a primary defense against Gram-negative bacteria. However, many bacterial pathogens have evolved mechanisms to resist MAC-mediated lysis. These mechanisms include: - **Capsule and Lipopolysaccharide (LPS) Structure**: The presence of a thick capsule or long O-antigen chains can prevent the insertion of the MAC into the outer membrane. - **Surface Proteases**: Some bacteria produce proteases that cleave and inactivate complement components, including C9. - **Complement Regulator Acquisition**: Some bacteria can bind host complement regulators, such as Factor H and C4b-binding protein, to their surface, which inactivates C3 and C5 and prevents MAC formation. The study of these bacterial evasion mechanisms is crucial for understanding the pathogenesis of infections and for the development of novel therapeutic strategies. --- ## 6. Pharmacogenomics, Drug Targets & Small-Molecule Inhibitors ### 6.1 C9 as a Therapeutic Target The terminal complement pathway, and C9 in particular, is an attractive target for therapeutic intervention in diseases where complement-mediated damage is pathological. These include: - **Age-Related Macular Degeneration (AMD)**: Chronic complement activation contributes to RPE cell death and choroidal neovascularization. Inhibiting MAC formation could slow disease progression. - **Autoimmune and Inflammatory Diseases**: Conditions such as rheumatoid arthritis, lupus nephritis, and myasthenia gravis involve complement-mediated tissue damage. - **Paroxysmal Nocturnal Hemoglobinuria (PNH)**: This disease is caused by a deficiency of GPI-anchored complement regulators (CD55 and CD59) on red blood cells, leading to complement-mediated hemolysis. Inhibiting C9 could prevent hemolysis. ### 6.2 FDA-Approved Drugs and Investigational Agents Currently, there are no FDA-approved drugs that specifically target C9. However, several drugs target other components of the complement pathway, and there is significant interest in developing C9-specific inhibitors. - **Eculizumab (Soliris)** and **Ravulizumab (Ultomiris)**: These are monoclonal antibodies that bind to C5, preventing its cleavage into C5a and C5b. By inhibiting C5, they block the formation of the MAC and are used to treat PNH, atypical hemolytic uremic syndrome (aHUS), and myasthenia gravis. These drugs indirectly inhibit C9 function by preventing its recruitment to the C5b-8 complex. - **Pegcetacoplan (Empaveli)** and **Iptacopan (Fabhalta)**: These are proximal complement inhibitors that target C3 and Factor B, respectively. They are used to treat PNH and other complement-mediated diseases. - **Investigational C9 Inhibitors**: Several companies are developing small molecules and antibodies that directly target C9. These agents are designed to bind to C9 and prevent its polymerization, thereby inhibiting MAC formation. Preclinical studies have shown that C9 inhibitors can effectively block complement-mediated hemolysis and inflammation. ### 6.3 Gene Therapy and Other Approaches Gene therapy approaches to treat C9 deficiency are theoretically possible but are not currently in clinical development. The C9 gene is relatively small, making it amenable to delivery via adeno-associated virus (AAV) vectors. However, the need for such therapy is limited, as C9 deficiency is rare and can be managed with prophylactic antibiotics and vaccination. ### 6.4 Pharmacogenomic Considerations The pharmacogenomics of C9 is an emerging field. Genetic variations in C9, such as the R95X mutation, can influence an individual's baseline complement activity and their response to complement-targeted therapies. For example, patients with C9 deficiency may not benefit from C5 inhibitors, as they already lack the ability to form a functional MAC. Conversely, patients with overactive complement due to specific C9 variants may be more responsive to C9 inhibitors. --- ## 7. Bioinformatic Resources & Database Accessions The following table provides key database accessions and resources for the C9 gene and protein. | **Database** | **Identifier** | **Description** | | :--- | :--- | :--- | | **NCBI Gene** | 735 | Gene ID for human C9 | | **Ensembl** | ENSG00000113648 | Ensembl gene ID for human C9 | | **UniProtKB** | P02748 | Primary protein sequence and annotation | | **RCSB PDB** | e.g., 5FMW, 6D5L | Experimentally determined structures of C9 and MAC | | **HGNC** | 1315 | HUGO Gene Nomenclature Committee entry | | **OMIM** | 120940 | Online Mendelian Inheritance in Man entry | | **ClinVar** | Various | Database of clinically relevant genetic variants | | **STRING** | P02748 | Protein-protein interaction networks | | **BioGRID** | 109119 | Biological General Repository for Interaction Datasets | | **Gene Ontology (GO)** | GO:0005576, GO:0006958, GO:0001869 | Extracellular region, complement activation, MAC pore formation | --- ## Related Clinical & Scientific Guides * [TARM1 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/tarm1-gene-structure-function-pathway) * [TRAC Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/trac-gene-structure-function-pathway) * [CFD Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/cfd-gene-structure-function-pathway) ## References [1] Wang, J., Wang, Q., Chen, Y., Wang, L., Zhao, A., & Sha, Z. (2020). Cloning, expression profile of the complement component C9 gene and influence of the recombinant C9 protein on peripheral mononuclear leukocytes transcriptome in half-smooth tongue sole (Cynoglossus semilaevis). *Fish and Shellfish Immunology*. [URL](https://www.semanticscholar.org/paper/352fa38f209fb9c8835baa68a5f63637d47aeb83) [4] Kong, C., Huang, Z.-A., Chen, J., Shi, Y.-H., & Lu, X.-J. (2013). Molecular cloning, sequence analysis and expression of ayu complement component C9 gene. *Dong wu xue yan jiu = Zoological research*. [URL](https://www.semanticscholar.org/paper/bae277105daab913a985cbdf4fe85814548568a0) [5] Li, L., Chang, M., & Nie, P. (2007). Molecular cloning, promoter analysis and induced expression of the complement component C9 gene in the grass carp Ctenopharyngodon idella. *Veterinary Immunology and Immunopathology*. [URL](https://www.semanticscholar.org/paper/83f6b392965ade47409029af9f966c6a2ecaf571) [6] Witzel-Schlömp, K., Spath, P., Hobart, M., Fernie, B., Rittner, C., Kaufmann, T., & Schneider, P. (1997). The human complement C9 gene: identification of two mutations causing deficiency and revision of the gene structure. *Journal of Immunology*. [URL](https://www.semanticscholar.org/paper/62fe776b1ef0bcc80dee9dbad0802e345317db4d) [7] Kira, R., Ihara, K., Watanabe, K., Kanemitsu, S., Ahmed, S., Gondo, K., Takeshita, K., & Hara, T. (1999). Molecular epidemiology of C9 deficiency heterozygotes with an Arg95Stop mutation of the C9 gene in Japan. *Journal of Human Genetics*. [URL](https://www.semanticscholar.org/paper/a9f7283bad9007b9c5fbae3b175b0d17cba51ded) [8] Kira, R., Ihara, K., Takada, H., Gondo, K., & Hara, T. (1998). Nonsense mutation in exon 4 of human complement C9 gene is the major cause of Japanese complement C9 deficiency. *Human Genetics*. [URL](https://www.semanticscholar.org/paper/1714dff6ae5a8115c296fb0101155838225bf1a2) [10] Witzel-Schlömp, K., Rittner, C., & Schneider, P. (2001). The human complement C9 gene: structural analysis of the 5' gene region and genetic polymorphism studies. *European Journal of Immunogenetics*. [URL](https://www.semanticscholar.org/paper/4f25af36728719b0f5194c2d34bdec74e2f46d00) [13] Tsujimura, R., Nishimukai, H., Okiura, T., Fukumori, Y., Tanabe, R., Orimoto, C., & Ueda, N. (2009). Typing of the nt343C>T (R95X) allele of the C9 gene by PCR-SSP and the allele frequency of R95X in five ethnic populations. *Legal Medicine*. [URL](https://www.semanticscholar.org/paper/9878110fe3374848e7c4beeba5f883ec4a71e475) [22] Mu, L., Qiu, L., Li, J., Bai, H., Lei, Y., Zeng, Q., Wang, L., Qi, W., Yin, X., & Ye, J. (2023). C9 regulates the complement-mediated cell lysis in association with CD59 to resist bacterial infection in a primary animal. *International Journal of Biological Macromolecules*. [URL](https://www.semanticscholar.org/paper/c59faac041a89b8f2a43b124902a15f778bb45df) [28] Baidya, A., Khatun, M., Mondal, R. K., Ghosh, S., Chakraborty, B., Mallik, S., Ahammed, S. M., Chowdhury, A., Banerjee, S., & Datta, S. (2022). Hepatitis B virus suppresses complement C9 synthesis by limiting the availability of transcription factor USF-1 and inhibits formation of membrane attack complex: implications in disease pathogenesis. *Journal of Biomedical Sciences*. [URL](https://www.semanticscholar.org/paper/23233641ea650ae65e64e510252f62636cd853b0) [34] Huang, H., Zhan, Y., Zong, H., Huang, C., Yang, F., Wei, Z., Qin, X., Crabbe, M., Wang, Y., & Zhang, X. (2025). Decoding hepatobiliary-specific immune gene patterns in gastrointestinal cancers via gene ontology fingerprints, multi-omics, and experimental integration. *Precision Clinical Medicine*. [URL](https://www.semanticscholar.org/paper/3b259caa6ee26c895ef9fbb8e892f4366bb40902) [36] Marazziti, D., Eggertsen, G., Fey, G., & Stanley, K. (1988). Relationships between the gene and protein structure in human complement component C9. *Biochemistry*. [URL](https://www.semanticscholar.org/paper/fed4704e4e5934e50701de39ae9cb6411ca1da1e) [37] Abbott, C., West, L., Povey, S., Jeremiah, S., Murad, Z., Discipio, R., & Fey, G. (1989). The gene for human complement component C9 mapped to chromosome 5 by polymerase chain reaction. *Genomics*. [URL](https://www.semanticscholar.org/paper/152356d722da8228627594577d3013dbaeea03c3) [40] Li, L., Shen, Y., Xu, X., Yang, W., & Li, J. (2021). Tracing and exploring the evolutionary origin and systematic function of fish complement C9. *Zeitschrift für Induktive Abstammungs- und Vererbungslehre*. [URL](https://www.semanticscholar.org/paper/86e4781bd63534f58be892b620ebbf024baf0b9b) [42] Hobart, M., Fernie, B., & Discipio, R. (1995). Structure of the human C7 gene and comparison with the C6, C8A, C8B, and C9 genes. *Journal of Immunology*. [URL](https://www.semanticscholar.org/paper/b27530486fe05b37a58a2926ccec176ed0865473) [44] Trapani, J. A., Kozak, C. A., Chintamaneni, C., Young, I., & Dupont, B. (1990). Genomic organization of the mouse pore-forming protein (perforin) gene and localization to chromosome 10. Similarities to and differences from C9. *Journal of Experimental Medicine*. [URL](https://www.semanticscholar.org/paper/4f8457f7f3f7e73551fabd7702eaab9b726c2b6b) [45] Würzner, R. (2003). Deficiencies of the complement MAC II gene cluster (C6, C7, C9): is subtotal C6 deficiency of particular evolutionary benefit? *Clinical and Experimental Immunology*. [URL](https://www.semanticscholar.org/paper/9db9b3899a4b06cbd5c5cb8908a79527b3a59efb) [46] Rogne, S., Myklebost, O., Stanley, K., & van Kessel, A. G. (1989). The gene for human complement C9 is on chromosome 5. *Genomics*. [URL](https://www.semanticscholar.org/paper/7b673f639630c41972ecfa2271f261e451a14fd6) [49] Guang-y, F. (2005). Effects of gene recombination sCR1 on the expression of C9 and Clusterin in acute spinal cord injury tissue of rats. *Scientific Publication*. [URL](https://www.semanticscholar.org/paper/c166c28840cda0966c51d47c02d57acc480b95e8) [50] Fu, Y.-w., Zhu, C.-k., & Zhang, Q.-Z. (2019). Molecular characterization and expression analysis of complement components C3 and C9 in largemouth bronze gudgeon (Coreius guichenoti) in response to Ichthyophthirius multifiliis infection. *Aquaculture*. [URL](https://www.semanticscholar.org/paper/377e37e6f82402c45392f10c1d499b3aff948503) [51] Hua, X.-t., Fan, K.-p., Zhang, Z., Li, X., Xia, Y.-q., Liu, P.-f., & Liu, Y. (2020). Characterization and expression analysis of the C8α and C9 terminal complement components from pufferfish (Takifugu rubripes). *Developmental and Comparative Immunology*. [URL](https://www.semanticscholar.org/paper/345aa92169bbf6a9f008ee009544bef8a897e980) [52] Kremlitzka, M., Geerlings, M., de Jong, S., Bakker, B., Nilsson, S. C., Fauser, S., Hoyng, C., de Jong, E. D., den Hollander, A. D., & Blom, A. (2018). Functional analyses of rare genetic variants in complement component C9 identified in patients with age-related macular degeneration. *Human Molecular Genetics*. [URL](https://www.semanticscholar.org/paper/c2fbfea974661c820fe5e156299b57f49c55e3fe) [53] Liu, R., & Moss, B. (2018). Vaccinia Virus C9 Ankyrin Repeat/F-Box Protein Is a Newly Identified Antagonist of the Type I Interferon-Induced Antiviral State. *Journal of Virology*. [URL](https://www.semanticscholar.org/paper/f1f9794f23092eac57a69b3f8c634ccd02f36bb5) [56] Hollborn, M., Ackmann