FKBP15 (FK506-binding protein 15) is an endosomal protein involved in intracellular membrane trafficking and protein sorting. Structurally, FKBP15 contains an ER retention signal and is located at endosomal membranes 1, though it appears catalytically inactive as a prolyl isomerase despite homology to active FKBP family members 1. Functionally, FKBP15 mediates membrane association through interaction with the Fam21 tail of the WASH complex 2, a regulator of endosomal tubule dynamics. FKBP15 depletion selectively inhibits β1-adrenergic receptor recycling from early endosomes 3, indicating a specialized role in GPCR sorting distinct from other trafficking pathways. The protein functions at the intersection of actin and microtubule dynamics, participating in early endosome transport during the transition between microfilament-based and microtubule-based movement 4. Disease relevance includes associations with ulcerative colitis, as FKBP15 is linked to endocytic pathways altered in inflammatory bowel disease 25. Additionally, FKBP15 phosphorylation is dysregulated in systemic lupus erythematosus patients, with reduced multisite phosphorylation correlating with disease activity 6. FKBP15 also appears as a fusion partner in NTRK2-rearranged gliomas, though its role in these malignancies remains unclear 7.