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FXR and LXR fall into a category of proteins termed orphan receptors; because of their lack of a defined function, and in the case of LXR, the lack of a defined ligand. FXR has been shown to bind a class of lipid molecules called farnesoids. LXR/RXR heterodimers have highest affinity for DR-4 DNA elements while FXR/RXR heterodimers bind IR-1 elements. Both LXR/RXR and FXR/RXR heterodimers retain their responsiveness to 9-cis retinoic acid. The steroid receptor superfamily acts through direct association with DNA sequences known as hormone response elements (HREs) and bind DNA as either homo- or heterodimers. The promiscuous mediator of heterodimerization, RXR, is the receptor for 9-cis retinoic acid, and dimerizes with VDR, TR, PPAR, as well as several novel receptors including LXR (also referred to as RLD-1) and FXR.
