To begin to distinguish between these two hypotheses, PK-15 clones over-expressing pig APOBEC3F were established and used . PK-15 and 293T cell lysates were used as negative controls. CEM and H9 were used as positive controls for APOBEC3G expression. A non-specific (but pan-species) band is shown as a protein loading control (marked by an asterisk). (B) Q-PCR data using genomic DNA prepared from 293T cells co-cultured with two independently derived APOBEC3G expressing PK-15 clones (G1 and G2, circles and triangles, respectively) or two vector control clones (V1 and V2, diamonds and squares, respectively). The experimental parameters are identical to those used in Figure 1B in transmission experiments. The former hypothesis was favored because pig APOBEC3F over-expression did not significantly interfere with PERV transmission (Online Figure S3B ). These results were further supported by PCR experiments showing that PERV could be amplified readily from 293T cells that had been co-cultured with PK-15 over-expressing pig APOBEC3F (unlike the APOBEC3G scenario; below).