FK506 binding protein, 12.6 kDa molecular weight (FKBP12.6), also called FKBP1B, is a peptidyl-prolyl isomerase that catalyzes the transition between cis- and trans- proline residues critical for proper folding of proteins. The immunosuppressant FK506 is a potent inhibitor of FKBP12.6. FKBP12.6 is localized in the sarcoplasmic reticulum associated with the Ryanodyne receptor RYR2. PKA-induced phosphorylation of RYR2 causes FKBP12.6 dissociation. In heart failure, RYR2 is hyperphosphorylated and has low levels of FKBP12.6 bound to it. Male FKBP12.6 knockout mice have cardiac hypertrophy and lethal exercise arrhythmias.