Murine and human APOBEC3 proteins inhibit endogenous retroviruses. (A) Rationale of the assay for detection of infection events by endogenous retroviruses in the presence of APOBEC3 proteins. The IAPE-D and HERV-K elements used in the assay are marked with the neo reporter gene – inserted in reverse orientation – and carry their own functional genes, except for the env gene which is supplied in trans, thus allowing only for single rounds of infection. Human 293T cells are co-transfected with the indicated expression vectors for APOBEC3 family members, the supernatants collected 2-days post-transfection to infect HeLa target cells, and infection events detected upon G418 selection. (B) Analysis of the activity of murine and human APOBEC3 proteins on the indicated endogenous retroviruses. Viral titers are given as percentages relative to a control (no apobec: expression vector with a nonfunctional hA3G; 622 and 549 G418R clones/ml for IAPE-D and HERV-K, respectively). Data are the means ± standard deviations (s.d.) for at least three independent experiments. Bottom: retrotransposition frequency of an active autonomous IAP element marked with a neo indicator gene for retrotransposition  in the presence of the corresponding APOBEC3 proteins; the assay was performed by cotransfection of HeLa cells with the marked IAP and APOBEC expression vector as previously described ; values are the means ± standard deviations (s.d.) for at least three independent experiments and are given as percentages relative to the control (no apobec; 1.3 × 10-3 G418R clones/cell).