Nuclear receptors are a class of cellular proteins that are responsible for sensing the ligand substances such as steroid and thyroid hormones and certain other molecules.In response to the ligands,these receptors work in concert with other proteins to regulate the expression of specific genes as transcription factors,thereby controlling the development,homeostasis,and metabolism of the organism.As the expression of numerous genes is regulated by nuclear receptors,the exogenous substances acting like intrinsic ligands can cause significant effects on the organism.Many of these regulated genes are associated with various diseases; therefore,nuclear receptors have been the molecular targets of many approved pharmaceuticals.Of the approximately 50 known human nuclear receptors,some receptors such as FXR,LXR,and PPAR bind to a number of metabolic intermediates such as fatty acids,bile acids and sterols.