RSAD2 (radical S-adenosyl methionine domain containing 2) is an interferon-stimulated gene encoding viperin, a multifunctional antiviral and immunomodulatory protein 1. Mechanistically, RSAD2 catalyzes conversion of cytidine triphosphate (CTP) to 3'-deoxy-3',4'-didehydro-CTP (ddhCTP) via a SAM-dependent radical mechanism, which acts as a chain terminator for viral RNA-dependent RNA polymerases, thereby broadly inhibiting DNA and RNA virus replication 2. Beyond antiviral functions, RSAD2 promotes TLR7/TLR9-dependent interferon-beta production and regulates CD4+ T-cell activation and Th2 differentiation 1. RSAD2 expression is markedly upregulated in M1 macrophages and in systemic lupus erythematosus (SLE) and primary Sjögren's syndrome patients 32. Disease relevance is substantial: elevated RSAD2 at the maternal-fetal interface in SLE causes pathogenic lipid accumulation impairing placental vascular development 4. In asthma, RSAD2 represents a key hub gene altered in patient immune cells 5. Clinically, RSAD2 inhibition by L-chicoric acid improves pregnancy outcomes in SLE models, suggesting therapeutic potential 4. RSAD2 is identified as a shared pathogenic gene in autoimmune diseases alongside STAT1 and IRF7 transcription factors 21.