To begin to genotype the infectious PERVs and to further probe the mechanism of PERV restriction by human APOBEC3G, the PERV pol gene DNA was amplified from human 293T cells, cloned and sequenced ( Figure 5A ; Online Figure S4 ). Twenty-nine and twentytwo sequences were analysed from APOBEC3G and control experiments, respectively. To minimize possible PCR biases, any sequence that was recovered multiple times was considered one event, unless it arose from independent experiments. These DNA sequence analyses revealed several important points.