APOBEC3G is a potent antiretroviral factor, which belongs to the APOBEC 
superfamily of cytidine deaminases. It deaminates cytidine to uridine in nascent 
minus-strand viral DNA, inducing G-to-A hypermutation in the plus-strand viral 
DNA. HIV-1 Vif protein overcomes the antiviral activity of APOBEC3G by targeting 
it for ubiquitin-dependent degradation. Recent accumulating evidences that other 
members of APOBEC proteins also show antiviral activity on a wide variety of 
viruses suggest that APOBEC family proteins play a crucial role in an antiviral 
defense as an innate immunity. Here, we review recent progress in research on 
APOBEC3 proteins.