The porcine kidney PK-15 fibroblast cell line and the swine testes ST-IOWA cell line were obtained from the ATCC and cultured in Dulbecco's modified Eagle's medium (Invitrogen) supplemented with 10% fetal bovine serum (Gemini), and 25 units/ml penicillin and 25 mg/ml streptomycin at 37uC and 5% CO 2 . Human embryonic kidney 293T and HeLa (A. Bielinsky, University of Minnesota) cell lines were grown under the same conditions. The T cell lines H9 and CEM (M. Malim, Kings College London) were cultured in RPMI-1640 supplemented with 10% fetal bovine serum (Gemini), and 25 units/ml penicillin and 25 mg/ml streptomycin at 37 uC and 5% CO 2 . Plasmids encoding human APOBEC3G, human APOBEC3G-E259Q and porcine APO-BEC3F were described previously [41] . The human APOBEC3G and porcine APOBEC3F cDNA sequences used here are identical to GenBank accession numbers, NM_021822 and NM_001097446, respectively.