Viperin (RSAD2) is an antiviral radical S‐adenosylmethionine (SAM) enzyme highly expressed in different cell types upon viral infection. Recently, it has been reported that the radical‐SAM activity of viperin transforms cytidine triphosphate (CTP) to its analogue 3ʹ‐deoxy‐3′,4ʹ‐didehydro‐CTP (ddhCTP). Based on biochemical studies and cell biological experiments, it was concluded that ddhCTP and its nucleoside form ddhC do not affect the cellular concentration of nucleotide triphosphates (NTPs) but act as replication chain terminators. However, our re‐evaluation of the reported data and our data indicate that ddhCTP is not an effective viral chain terminator but depletes cellular nucleotide pools and interferes with mitochondrial activity to inhibit viral replication. Our analysis is consistent with a unifying view of the antiviral and radical‐SAM activities of viperin.
