Mechanism of APOBEC3G encapsidation. Four different scenarios can be envisioned: (1) APOBEC3G is packaged non-specifically. This possibility seems unlikely given the known affinity of APOBEC3G to viral Gag proteins and viral and cellular RNAs. (2) APOBEC3G is packaged through interaction with Gag in an RNA-independent manner. Such a mechanism was initially proposed as discussed in the text. (3) APOBEC3G interacts with host RNA; in particular 7SL RNA was proposed to mediate encapsidation of APOBEC3G. (4) APOBEC3G is packaged through specific interaction with viral genomic RNA. This model is our favorite and is supported by the observation that point mutations in the 5' untranslated region of the viral genome can severely affect APOBEC3G packaging without affecting 7SL RNA encapsidation [6, 10]. This model is also consistent with the data reported by Bach et al. .