Retroviral integrase (IN) is an enzyme produced by a retrovirus (such as HIV) that integrates (forms covalent links between) its genetic information into that of the host cell it infects.

[2] All retroviral IN proteins contain three canonical domains, connected by flexible linkers:[3][4] Crystal and NMR structures of the individual domains and 2-domain constructs of integrases from HIV-1, HIV-2, SIV, and Rous Sarcoma Virus (RSV) have been reported, with the first structures determined in 1994.

[8] Human foamy virus (HFV), an agent harmless to humans, has an integrase similar to HIV IN and is therefore a model of HIV IN function; a 2010 crystal structure of the HFV integrase assembled on viral DNA ends has been determined.

[10] After integration, the viral gene expression and particle production may take place immediately or at some point in the future, the timing depends on the activity of the chromosomal locus hosting the provirus.

[6][11] In November 2005, data from a phase 2 study of an investigational HIV integrase inhibitor, MK-0518, demonstrated that the compound has potent antiviral activity.