Immunophilins are a highly conserved family of cis-trans peptidyl-prolyl isomerases which bind to and mediate the effects of immunosuppressive drugs such as Cyclosporin, FK506 and Rapamycin. FKBP12.6, also known as FK506-binding protein 1B, is a 108 amino acid immunophilin belonging to the FKBP-type PPIase family. Subcellularly localized to the cytoplasm, FKBP12.6 binds to RyR in cardiac muscle sarcoplasmic reticulum and possibly plays a unique physiological role in excitation-contraction coupling in cardiac muscle. FKBP12.6 also catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides. Ubiquitously expressed, FKBP12.6 is found at highest levels in brain and thymus. FKBP12.6 is expressed as two isoforms produced by alternative splicing.