Stable APOBEC3G-or vector control-expressing PK-15 cell lines were constructed by transfection using FuGENE6 according to the manufacturer's protocol (Roche) or by electroporation (BioRad, 250V, 950 mFa). Clones were selected using growth medium containing 1 mg/ml G418 (Roche), and APOBEC3G expressing clones were identified by immunoblotting using a polyclonal antibody toward human APOBEC3G (J. Lingappa, University of Washington). All PK-15 clones were maintained in growth medium supplemented with 250 mg/ml G418 to ensure stable expression.