The second key step in addressing our experimental question was isolating PK-15 clones that stably expressed human APOBEC3G. Clones expressing human APOBEC3G cDNA or an empty vector control were established in parallel ( Figure 2A ). Immunoblotting identified clones with APOBEC3G levels similar to those in known APOBEC3G-expressing human T cell lines CEM and H9, which are non-permissive for growth of Vif-deficient HIV-1 [19] . Although it was impossible to achieve a physiologic expression level, the comparative immunoblot at least ensured that the levels of APOBEC3G were equal or lower than those present in wellstudied, non-permissive human T cell lines.