FK506 binding protein 5 (FKBP51, also known as FKBP5) is a member of the immunophilin (IMM) protein family of intracellular proteins. The signature domain of the IMM family is the peptidyl-prolyl-cis/trans-isomerase (PPIase) domain, which is in turn the drug binding domain. IMMs are classified by their ability to bind immunosuppressant drugs – CyPs (cyclophilins) bind cyclosporine A (CsA), and FKBPs (FK506-binding pro-teins) bind FK506 (Pratt and Toft 1997; Kang et al. 2008). In addition to the PPIase domain, there are three additional domains – the nucleotide-binding domain, (also called FKBD2 in FKBP proteins) where ATP binds, the calmodulin-binding domain, a poorly characterized domain able to interact with calmodulin, and tetratricopeptide repeat (TPR) domains, sequences of 34 amino acids repeated in tandem through which FKBPs bind to the HSP90 C-terminal sequence MEEVD (Davies et al. 2005; Wu et al. 2004). Mass spectrometry analysis showed that FKBP51 (FKBP5) and FKBP52 (FKBP4) form analogous complexes with GR:HSP90:STIP1:HSP70:ATP (Ebong IO et al. 2016). Binding of FKBP51 (FKBP5) and other immunophilins may weaken the association of TPR domain containing protein STIP1 with HSP90 complex (Li et al. 2011).