Viral infectivity factor, or Vif, is an accessory protein found in HIV and other lentiviruses.

Vif1 inhibits the cellular protein APOBEC3G from entering the virion during budding from a host cell by targeting it for proteasomal degradation.

[9][10][11] But recent studies with the use of metabolic labelling demonstrated that serine/threonine phosphorylation of Vif1 and A3G is not required for the interaction of Vif1 with A3G for Vif dependent degradation of A3G and the antiviral activity of A3G.

[12] However, a recent study by Raja et al has shown that Host AKT-Mediated phosphorylation of HIV-1 Vif at Thr20 stabilizes it to enhance APOBEC3G degradation and potentiate HIV-1 infectivity.

[11] Vif2 is only about ~30% identical at the amino acid level to Vif1, a result of the evolutionary separation in different source species of the two viruses (see Subtypes of HIV).

[5] A 2018 review lists 17 small molecules capable of stopping viral replication by Vif inhibition, and categorized them into the functional categories of Vif multimerization targeting, A3G-Vif-targeting (two subcategories by the binding interface disrupted), Vif-EloC targeting, and A3G-upregulating.

[17][18] The mamallian APOBEC3 enzymes are in an arms race with Vifs found in those viruses, actively evolving and diversifying to escape inactivation.