FK506 binding protein, 25 kilodalton molecular weight (FKBP25), also called FKBP3, is a peptidyl-prolyl isomerase that catalyzes the transition between cis- and trans-proline residues critical for proper folding of proteins. The macrolide immunosuppressant rapamycin is a potent inhibitor of FKBP25. FKBP25 is localized in the nucleus, where it alters the activity of histone deacetylases and transcription factors. FKBP25 is found in many tissues, including the brain, thymus, and spleen.