Cyclophilin A, also called Peptidylprolyl Isomerase A, PPIA, CYPA, and CYPH, was originally characterized for its ability to catalyze the transition between cis-and trans-proline residues critical for proper folding of proteins. 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.