The liver X receptor (LXR) is a nuclear hormone receptor that is involved in the regulation of fatty acid and cholesterol metabolism. Nuclear hormone receptors constitute a family of specialized proteins in the nucleus that can switch on individual genes in response to the changing needs of the organism. These proteins contain two functional units: (1) a deoxyribonucleic acid (DNA)–binding domain that binds to target DNA sequences within the promoter region of genes that regulate many aspects of vertebrate metabolism and (2) a ligand-binding domain, to which hormones or other small molecules (ligands) attach. Binding of the ligand activates the receptor complex, displaces corepressor proteins from the promoters, and helps recruit coactivator proteins that link the receptor complex to the basic transcription machinery of the gene. This activates the production of multiple copies of its messenger RNA (mRNA) that, in turn, stimulate protein synthesis.