Interferon-inducible antiviral protein which plays a major role in the cell antiviral state induced by type I and type II interferon (PubMed:31812350). Catalyzes the conversion of cytidine triphosphate (CTP) to 3'-deoxy-3',4'-didehydro-CTP (ddhCTP) via a SAM- dependent radical mechanism (PubMed:29925952, PubMed:30872404). In turn, ddhCTP acts as a chain terminator for the RNA-dependent RNA polymerases from multiple viruses and directly inhibits viral replication (PubMed:29925952). Therefore, inhibits a wide range of DNA and RNA viruses, including human cytomegalovirus (HCMV), hepatitis C virus (HCV), west Nile virus (WNV), dengue virus, sindbis virus, influenza A virus, sendai virus, vesicular stomatitis virus (VSV), zika virus, and human immunodeficiency virus (HIV-1) (PubMed:29925952, PubMed:30587778, PubMed:30934824, PubMed:31921110). Also promotes TLR7 and TLR9-dependent production of IFN-beta production in plasmacytoid dendritic cells (pDCs) by facilitating 'Lys-63'-linked ubiquitination of IRAK1 by TRAF6 (PubMed:30872404). Plays a role in CD4+ T-cells activation and differentiation. Facilitates T-cell receptor (TCR)- mediated GATA3 activation and optimal T-helper 2 (Th2) cytokine production by modulating NFKB1 and JUNB activities. Can inhibit secretion of soluble proteins. {ECO:0000269|PubMed:11752458, ECO:0000269|PubMed:16108059, ECO:0000269|PubMed:16982913, ECO:0000269|PubMed:17686841, ECO:0000269|PubMed:18005719, ECO:0000269|PubMed:19074433, ECO:0000269|PubMed:29925952, ECO:0000269|PubMed:30587778, ECO:0000269|PubMed:30872404, ECO:0000269|PubMed:30934824, ECO:0000269|PubMed:31812350, ECO:0000269|PubMed:31921110}. This is the function of RSAD2 (radical S-adenosyl methionine domain containing 2, ENSG00000134321).