Several observations indicate that mA3 functions in antiviral defense: mA3 inhibits infection by several viruses including HIV-1 and mouse retroviruses such as mouse mammary tumor virus (MMTV), intracisternal A-particles (IAPs) and MusD endogenous retroviruses [12]–[14]; mA3 knockout mice are more susceptible to MMTV infection and tumorigenesis [15]; endogenous retroviruses (ERVs) of MLV in the sequenced Mus genome show modifications consistent with APOBEC3 activity [16]. Here, PNMA3 is linked to infection.