Nuclear retinoic acid "orphan" receptors: progress towards an understanding of the new complexities in retinoid metabolism.

A new retinoic acid-related receptor has been identified. It is called the retinoid X receptor and is distinct from the retinoic acid receptors, alpha, beta and gamma. It represents an "orphan" receptor because its function and ligand are unknown. What is known is that it is structurally related to the steroid receptor genes. The ligand to which the retinoic acid X receptor binds has recently been determined to be either retinoic acid itself or another metabolically derived molecule. It is speculated that retinoic acid could serve as a precursor that is metabolized to a more active form as is the case for vitamin D and estrogen (24). The elucidation of an alternative pathway in retinoid metabolism will increase our understanding of the scope of retinoid action.