Melatonin has been reported to be the endogenous ligand for ROR-α while CGP 52608 has been identified as a ROR-α selective synthetic ligand. However X-ray crystallographic (PDB: 1n83 and 1s0x) and functional data both suggest that cholesterol or a cholesterol derivative may be the endogenous ligand.
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^Hirose T, Smith RJ, Jetten AM (1994). "ROR gamma: the third member of ROR/RZR orphan receptor subfamily that is highly expressed in skeletal muscle". Biochem. Biophys. Res. Commun.205 (3): 1976–83. doi:10.1006/bbrc.1994.2902. PMID7811290.