FKBP (FK506-binding protein) is one of two major immunophilins and most of FKBP family members bind FK506 and show peptidylprolyl cis/trans isomerase (PPIase) activity. Small size FKBP family members contain only FK506-binding domain, while FKBPs with large molecular weights possess extra domains such as tetratricopeptide repeat domains, calmodulin binding and transmembrane motifs. FKBPs are involved in several biochemical processes including protein folding, receptor signaling, protein trafficking and transcription. FKBP family proteins play important functional roles in the T-cell activation, when complexed with their ligands.

FK506-binding proteins 1a and 1b (FKBP1a/1b) are immunophilin proteins that bind the immunosuppressant agent FK506 and AY 22989. FKBP12 is a ubiquitous abundant protein that acts as a receptor for FK506, binds tightly to intracellular calcium release channels and to the transforming growth factor β (TGF-β) type I receptor.