Standard reverse transcription (RT)-PCR reactions were performed using RNA prepared from PK-15 cells (TRIzol protocol, Invitrogen), M-MLV reverse transcriptase was used for cDNA synthesis using an oligo dT primer (Ambion) and Taq polymerase was used for PCR (Roche). The primers specific to pig APOBEC3F were 59-TGG TCA CAG AGC TGA AGC AG and 59-TTG TTT TGG AAG CAG CCT TT (175 bp). The semi-nested primer set used to detect plasmid-expressed pig APOBEC3F was 59-CCA AGG AGC TGG TTG ATT TC (exon 6, reaction 1), 59-CTG GAG CAA TAC AGC GAG AG (exon 7, reaction 2) and 59-TAG AAG GCA CAG TCG AGG, with the latter being vector specific (319 bp and 190 bp products, respectively). The mammalian beta-actin primers were 59-CCT TCA ATT CCA TCA TGA AGT G and 59-CCA CAT CTG CTG GAA GGT (236 bp). These primers amplify equally well a 236 bp beta-actin fragment from all mammals tested, including pigs and humans (e.g., Online Figure S3 ).