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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.FKBP12:FK506 complexes inhibit calcineurin, a calcium/calmodulin-dependent serine/threonine phosphatase which blocks T-cell activation by preventing lymphokine gene transcription. FKBP12 also plays a role in intracellular calcium regulation by associating with three types of calcium release channel complexes, cardiac and skeletal ryanodine receptors and the inositol 1,4,5-trisphosphate receptor. In interactions with members of the TGF beta family type I receptors, FKBP12 has been shown to exert an inhibitory effect on the signaling pathway.