 Many of these regulated genes are associated with various diseases, which explains why the molecular targets of approximately 13% of U.S. Food and Drug Administration (FDA) approved drugs target nuclear receptors.A number of nuclear receptors, referred to as orphan receptors, have no known (or at least generally agreed upon) endogenous ligands.Some of these receptors such as FXR, LXR, and PPAR bind a number of metabolic intermediates such as fatty acids, bile acids and/or sterols with relatively low affinity.