C2 and systemic lupus erythematosus: These multilayer processes in SLE pathogenesis involve genes participating in apoptosis (TNFRSF21, IKBKG, IKBKB, BCL2L11, BAK1, TRAF3, IRF1, IRF3, IRF4, IRF5, IRF7, CCND2, PYCARD), regulation of ribonuclease activity (OAS1, OAS2, OAS3, OASL), complement activation (C1QB, C2, C3, C4B, CFB, ITGAM, ITGAX), and Fcγ receptor-mediated phagocytosis (FCGR2A, FCGR3A, FCGR3B, PTPRC, LYN, NCF1).