FKBP5/FKBP51 is an immunophilin and target of FK506, a macrolide immunosuppressive drug derived from Streptomyces tsukubaensis. Like other FK506 binding proteins, FKBP5/FKBP51 exhibits peptidyl-prolyl cis-trans isomerase activity. The most studied role of FKBP5/FKBP51 surrounds its involvement in the regulation of steroid receptor signaling. In association with the molecular chaperones Hsp90 and Hsp70, FKBP5/FKBP51 is able to influence steroid-hormone binding affinity and transcriptional activity. Recently, FKBP5/FKBP51 has become of interest in the field of psychiatry due to its potential role in cortisol regulation and its relation to mood disorders.