The peptidyl-prolyl cis-trans isomerases, also known as cyclophilins, share this domain of about 109 amino acids. Cyclophilins have been found in all organisms studied so far and catalyse peptidyl-prolyl isomerisation during which the peptide bond preceding proline (the peptidyl-prolyl bond) is stabilised in the cis conformation. Mammalian cyclophilin A (CypA) is a major cellular target for the immunosuppressive drug cyclosporin A (CsA). Other roles for cyclophilins may include chaperone and cell signalling function [1].

1: The cyclophilins. Wang P, Heitman J; Genome Biol 2005;6:226. PMID:15998457