FKBP is one of the two main immunophilic proteins. Most members of the FKBP family bind to FK506 and exhibit peptide-proline cis-trans-isomerase (PPIase) activity. The small molecule FKBP family members only contain the FK506 binding domain, while the large molecular weight FKBP has additional domains such as tetrapeptide repeat domain, calmodulin binding domain and transmembrane motif. FKBPs are involved in a variety of biochemical processes, including protein folding, receptor signal transduction, protein transport and transcription. FKBP family proteins play an important role in T cell activation when combined with ligand.