During the course of these experiments, we reported some of the activities of pig APOBEC3F [41] . It could strongly inhibit the replication of HIV (regardless of Vif) and modestly inhibit the replication of MLV, a gamma-retrovirus phylogenetically related to PERV. Therefore, we wondered whether pig APOBEC3F was expressed in PK-15 and, if so, whether PERV resists this cellular defense. To begin to address this possibility, RT-PCR was used to test PK-15 cells for pig APOBEC3F expression. Pig APOBEC3F mRNA was detected readily (Online Figure S3A ). Full cDNA sequencing revealed that the predicted APOBEC3F protein of PK-15 cells was 98% identical to the variant we reported previously [41] . Eight amino acid differences were found, but both the PK15 and the previously reported APOBEC3F sequences were represented in pig genomic DNA sequences suggesting that these may be breed-specific polymorphisms (R.S.L. and R.S.H., manuscript in preparation). These observations indicated that either PERV resists the endogenous APOBEC3F protein of its host or that the level of suppression by pig APOBEC3F is not sufficient to inhibit PERV transmission.