The FKBPs, or FK506 binding proteins, constitute a family of proteins that have prolyl isomerase activity and are related to the cyclophilins in function, though not in amino acid sequence.

[2] FKBP1A (also known as FKBP12) is notable in humans for binding the immunosuppressant molecule tacrolimus (originally designated FK506), which is used in treating patients after organ transplant and patients with autoimmune disorders.

[3] Tacrolimus has been found to reduce episodes of organ rejection over a related treatment, the drug ciclosporin, which binds cyclophilin.

[4][5] Both the FKBP-tacrolimus complex and the cyclosporin-cyclophilin complex inhibit a phosphatase called calcineurin, thus blocking signal transduction in the T-lymphocyte transduction pathway.

[6] This therapeutic role is not related to its prolyl isomerase activity.