The effect of TRIM5 on retroviral transduction. In the absence of a restrictive TRIM5α orthologue (No TRIM5), the retroviral genomic RNA is reverse transcribed, translocated to the nucleus and integrated. In the presence of a restrictive allele of TRIM5, retroviral infection is blocked at the level of reverse transcription and, probably, also at the level of nuclear import. According to some investigators, TRIM5 binding to the incoming virion capsid is sufficient to disrupt infection (sufficiency model). According to other investigators, the antiviral effects of TRIM5 require as yet to be identified, cellular cofactors (cofactor model).