The story of a poacher-turned-gamekeeper: a twist in animal evolution.

A retrovirus is an RNA virus. In a host cell, it typically replicates with the help of an enzyme reverse transcriptase to produce DNA from its RNA genome. The resulting DNA is incorporated into the genome of the host by an in tegrase enzyme. The v i rus thereaf te r rep l ica tes as par t o f the hos t DNA. Ret rov i ruses a re genera l ly enve loped v i ruses and the i r genomes commonly contain three open reading frames (ORFs) that encode for proteins found in the mature virus. The proteins are: group-specific antigen (gag) that codes for core and s t ruc tura l pro te ins of the v i rus , polymerase (pol ) tha t codes for reverse t ranscr iptase, protease and integrase, and envelope (env ) that codes for the retroviral coat proteins .