Cyclophilins are a highly conserved family of peptidylprolyl cis-trans-isomerases (PPIA) that are targets of the immunosuppressant drug cyclosporin A (CsA) (1,2). The complex of cyclophilin and CsA can bind to and inhibit calcineurin which leads to inhibition of the transcription factor NFAT and decreased production of cytokines (3,4). As isomerases, cyclophilins have been proposed to aid in protein folding. Cyclophilin A can bind to the p55 Gag protein of HIV and appears necessary for HIV infection (5,6). There is also some evidence that cyclophilins have nuclease activity and play a role in apoptosis (7).
