APOBEC3G, or A3G for short, is an enzyme that was previously known to fight HIV by inhibiting wild type Viral infectivity factor (Vif). It was the first enzyme that turned cytidine to uridine to be discovered. Some researchers now believe that it acts as a "double agent" by actually facilitating Vif depending on the amounts of DNA deaminase activity. According to the researchers studying this enzyme "The simplest answer might be the little DNA deaminase activity benefits the virus and a high level of activity destroys the virus" (Smith). This enzyme can be antiviral by inhibition of VIF or an HIV facilitator by bringing diversity to the genome of the virus which in turn gives new versions of the virus drug resistance properties.
APOBEC3G is a deaminase, which removes an amino radical from an amino acid or a compound. A3G deaminase-dependent antiviral activity hinders Vif by changing its genetic code and not allowing reverse transcription to take place.
APOBEC3G: a Double Agent in Defense
Harold C. Smith