An ultimate application of the technology described here raises the potential problem that human APOBEC3G may not be subjected to (proper) post-translational regulation in pig cells and it may therefore promote carcinogenesis. Expression of human APOBEC3G in a heterologous system has been shown to trigger elevated levels of genomic C/G-to-T/A transition mutation [44] . Therefore, to help mitigate this risk (in addition to establishing clones that expressed relatively modest APOBEC3G levels; above), we asked whether the predominantly cytoplasmic localization pattern of human APO-BEC3G would be maintained in PK-15 and in a swine testes cell line, ST-IOWA ( Figure 3 ; compare with other APOBEC3G reports [18, 35, 41] ). Unlike some other APOBEC3 proteins such as human APOBEC3B, which is mostly nuclear, both human APOBEC3G and pig APOBEC3F appeared predominantly cytoplasmic in either the human or the pig cell lines (Figure 3 ; [28, 31, 41] ). Both proteins also appeared to concentrate in cytoplasmic punctae, which varied in number and were apparent in some of the cells regardless of species of origin (described previously for APOBEC3G; e.g., [45, 46] ). Overall, these near-identical localization patterns suggested that human APOBEC3G is not aberrantly regulated in pig cells and, interestingly, that these proteins might be subjected to the same cellular regulatory mechanism(s).