The function of C9 (complement C9, Ensembl gene identifier ENSG00000113600) is as follows. Pore-forming component of the membrane attack complex (MAC), a multiprotein complex activated by the complement cascade, which inserts into a target cell membrane and forms a pore, leading to target cell membrane rupture and cell lysis (PubMed:22832194, PubMed:26841837, PubMed:26841934, PubMed:27052168, PubMed:30552328, PubMed:6177822, PubMed:9212048, PubMed:9634479). The MAC is initiated by proteolytic cleavage of C5 into complement C5b in response to the classical, alternative, lectin and GZMK complement pathways (PubMed:39914456, PubMed:39814882, PubMed:9212048, PubMed:9634479). The complement pathways consist in a cascade of proteins that leads to phagocytosis and breakdown of pathogens and signaling that strengthens the adaptive immune system (PubMed:9212048, PubMed:9634479). Constitutes the pore- forming subunit of the MAC complex: during MAC assembly, C9 associates with the C5b8 intermediate complex, and polymerizes to complete the pore (PubMed:26841934, PubMed:30111885, PubMed:30552328, PubMed:34752492, PubMed:4055801, PubMed:6177822). {ECO:0000269|PubMed:22832194, ECO:0000269|PubMed:26841837, ECO:0000269|PubMed:26841934, ECO:0000269|PubMed:27052168, ECO:0000269|PubMed:30111885, ECO:0000269|PubMed:30552328, ECO:0000269|PubMed:34752492, ECO:0000269|PubMed:39814882, ECO:0000269|PubMed:39914456, ECO:0000269|PubMed:4055801, ECO:0000269|PubMed:6177822, ECO:0000269|PubMed:9212048, ECO:0000269|PubMed:9634479}.