The retroviral life cycle. A schematic view of early and late stages of the retroviral replication cycle is represented. Examples of cellular factors interfering with early steps are indicated: Lv1/Ref1; CEM15, also known as APOBEC3G (apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like-3G) ; Fv2; Fv1. The question marks indicates the exact step affected by the restriction factors has not precisely been determined. Lv1 and Ref1 block incoming particles before reverse-transcription whereas Fv1 and Fv2 act at a stage between reverse-transcription and integration. See text for detailed discussion. Abbreviations: RTC, reverse transcription complex; PIC, pre-integration complex.