TRIM5 and infection: In the present study, we establish that allelic variation in the rhesus macaque TRIM5 gene results in differences in susceptibility to infection and viral replication in the early stages of cross-species transmission of SIVsm and that emergence of pathogenic SIVmac in rhesus macaques required adaptations in the viral capsid protein (CA) to overcome suppression by two distinct types of TRIM5 allele.