Peptidyl-prolyl cis-trans isomerase FKBP1B（FKBP1B) belongs to the immunophilin protein family which has a role in immunoregulation and basic cellular processes involving protein folding and trafficking. FKBP1B is a cis-trans prolyl isomerase which binds the immunosuppressants FK506（tacrolimus) and rapamycin（sirolimus). FKBP1B is extremely similar to the FK506-binding protein 1A. The physiological role of FKBP1B is thought to be in excitation-contraction coupling in cardiac muscle.
