OBSOLETE. Any member of a family of receptors that includes the major FK506 binding protein FKBP and cyclophilin. These two proteins are unrelated in amino-acid sequence, but both possess peptidyl-prolyl isomerase activity which is blocked by immunosuppressants that block signal-transduction pathways leading to T cell activation such as FK506 and rapamycin, which block FKBP, or cyclosporin A, which blocks cyclophilin.