Immunophilins are a family of soluble receptors capable of binding to one of two major immunosuppressant agents; cyclosporin A (CsA) or FK506. Proteins that bind FK506 are termed FK506 Binding Proteins (FKBPs) and those that bind cyclosporin A are called cyclophilins (CyP). Immunophilins function as cis-trans peptidyl-prolyl isomerases (PPIase) whose activity is inhibited by their respective immunosuppressant compounds. Thus, immunophilins accelerate folding of some proteins both in vivo and in vitro by catalyzing slow steps in the initial folding and rearrangement of proline-containing proteins.