The immunophilins are a highly conserved family of cis-trans peptidyl-prolyl isomerases that bind to and mediate the effects of immunosuppressive drugs, such as cyclosporin, FK506 and rapamycin (1). Several related immunophilins, FKBP12, FKBP51 and FKBP52, are characterized as cytosolic FK506-binding proteins, and following ligand binding, they functionally inhibit the phosphatase activity of calcineurin (2,3). The ubiquitously expressed FKBP12 also associates with the cytoplasmic domain of the TGF?type I receptor, where it stabilizes the inactive conformation of the receptor and blocks the activation of the TGF?pathway (4). FKBP51 and FKBP52 are two highly related proteins (5,6). FKBP51 is predominantly expressed in T cells and is induced by glucocorticoids (5). FKBP51 mediates the effects of FK506 and rapamycin by inhibiting intracellular calcineurin activity, and by blocking T-cell activation and proliferation (7). FKBP52, known also as FKBP-59 or heat shock protein 56, is expressed in a variety of tissues and can also associate with the heat shock protein (hsp90) in mature steroid receptor complexes (6,8).