The cyclophilins are a conserved class of proteins that bind the immunosuppressive drug cyclosporin A (CsA) with high affinity. CsA blocks helper T-cell activation at a step between T-cell receptor stimulation and the transcriptional activation of cytokine genes. Cyclophilins from many species possess peptidyl-prolyl cis-trans isomerase (PPIase) activity that is blocked by CsA and therefore may be relevant in CsA-mediated immunosuppression.