FKBP15, also known as FKBP133, is a member of the FK506-binding protein family, a group of proteins initially identified as immunophilins, targets for the immunosupressant drugs FK506 and Rapamycin. FKBP15 is expressed in the developing nervous system and contains a domain similar to Wiskott-Aldrich syndrome protein homology region 1 (WH1) in addition to the FK506-binding protein motif. FKBP15 is distributed along the axonal shafts and partially co-localizes with F-actin in the growth cones of dorsal root ganglion neurons; overexpression of FKBP15 resulted in the number of filopodia in transfected neurons, suggesting that FKBP15 modulates growth cone behavior. FKBP15 has also been shown to associate with both microtubules and the actin filament systems and disruption of its expression by RNAi resulted in delayed transport of early endosomes in HeLa cells indicating that FKBP15 is also involved in the transport of early endosomes. At least three isoforms of FKBP15 are known to exist.