We further noted that it is highly unlikely that another resident APOBEC3 protein contributes to PERV restriction, because genomic DNA sequencing showed that pigs have only one APOBEC3 gene, APOBEC3F (R.S.L. and R.S.H., manuscript in preparation). These observations combined to indicate that PERV is resistant to the endogenous APOBEC3 protein of its host. In hindsight, this was not particularly surprising given the emerging trend that (successful) retroviruses are selected in part by their ability to evade the APOBEC3 proteins of their host species (see Introduction).