Peptidyl-prolyl cis-trans isomerase A is a member of the peptidyl-prolyl cis-trans isomerase (PPIase) family, which catalyzes the cis-trans isomerization of proline imidic peptide bonds. Cyclophilin A regulate many biological processes, including intracellular signaling, transcription, inflammation, and apoptosis. Cyclophilin is also incorporated into many viruses, including HIV1, where it has been speculated to be involved in functions such as viral assembly and infectivity. The immunosuppressive activity of cyclosporins has been correlated with their ability to form complexes with cyclophilins that inhibit calcineurin phosphatase activity and prevent incorporation of cyclophilin into viral particles. The cyclosporin/cyclophilin complex selectively binds and inactivates calcineurin, making it a useful inhibitor for studying calcineurin activity.