 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.This therapeutic role is not related to prolyl isomerase activity.FKBP (FKBP1A) does not normally form a dimer but will dimerize in the presence of FK1012, a derivative of the drug tacrolimus (FK506).