Does not have 2'-5'-OAS activity, but can bind double-stranded RNA (PubMed:12396720, PubMed:27663720). The full-length protein displays antiviral activity against flaviviruses such as west Nile virus (WNV) via an alternative antiviral pathway independent of RNase L (PubMed:12080145, PubMed:16371364, PubMed:17904183, PubMed:27663720). The truncated form of the protein lacks antiviral activity (PubMed:12080145, PubMed:16371364, PubMed:17904183). Interacts with OSBPL1A and ABCF3. Highly expressed in lung, spleen and thymus (PubMed:12396720, PubMed:12186974). Also detected at lower levels in heart, kidney, liver, lung, skeletal muscle, testes, uterus and ovaries (PubMed:12396720, PubMed:12186974, PubMed:27663720). Detected 1 week after birth in developing skin, testes, ovary, kidney and lung. Up-regulated by the type I interferon IFNB1, in a STAT2-dependent manner (PubMed:22305621). Induced by polyinosinic:polycytidylic acid (poly I:C) (PubMed:12396720, PubMed:27663720). A nonsense mutation in exon 4 results in a premature stop codon leading to a truncated form which determines the resistant/susceptible phenotype of the mice to flaviviruses. Mice encoding the full-length protein show a flavivirus resistance phenotype (Flv(r)), whereas mice encoding a C-terminally truncated protein show a flavivirus susceptible phenotype (Flv(s)). Belongs to the 2-5A synthase family.