Species-specific inhibition of APOBEC3 (A3) proteins by Vif. HIV-1 and SIV can incorporate A3 proteins from all these species into virions. However, Vif from HIV-1 (VifHIV, brown), can only inhibit A3 proteins from humans and new world monkeys) but not those from old world monkeys and mice. In contrast, Vif from SIVmac (VifSIV, green) can inactivate A3 proteins from old and possibly new world monkeys, but not from humans and mice). In this drawing, stick figures represent sources of A3 proteins and not targets of infection.