RESUMEN
During their evolution, mammals have developed cellular factors interfering with retroviral replication, known as « restriction factors ¼. The prototype of these factors, Fv1, was characterized in the late 60's and blocks MLV infection. Some Fv1-like factors interfering with complex retroviruses, including HIV, have recently been discovered in primate cells. These restriction factors are referred to as Ref1, which blocksMLVreplication in human cells, and Lv1, which blocks the infection of non-human primate cells by various retroviruses, including MLV and HIV. These factors are all saturable by an excess of virus, target the viral capsid and interfere with an early step of viral replication. Lv1 and Ref1 have recently been found to be species-specific variants of a single protein called TRIM5α, a member of the TRIM protein family. The mechanism of action of these factors is still unknown. The existence of natural inhibitors of retroviral infection raises new hopes for the development of therapeutic tools against HIV infection.