FK506 Binding Proteins (FKBPs) are intracellular receptors for the immuno-suppressive drug FK506. The FKBP/FK506 complex exerts its immunosuppressive effects by inhibiting calcineurin, a calcium- and calmodulin-dependent serine/threonine phosphatase that functions as a critical signaling molecule during T-cell activation. FKBP38, also known as FKBP8, is a 355aa protein with a calculated molecular weight of 38.7kD and an apparent molecular mass of ~60-64kD in SDS-PAGE. FKBP38 binds to and inhibits calcineurin even in the absence of FK506, indicating that FKBP38 is a constitutively active inhibitor of calcineurin. Additionally, FKBP38 co-immunoprecipitates with Bcl-2 and Bcl-xL, suggesting that FKBP38 may regulate apoptosis by anchoring Bcl-2 and Bcl-xL to mitochondrial membranes. Mouse FKBP38 shares 94% and 87% sequence identity with human and rat FKBP38, respectively.