Figure 3: Endogenous and xenobiotic lipophilic ligands. Various xenobiotics and endogenous lipids are able to activate several nuclear receptors, which in turn control have the physiological role of controlling the intrahepatic and extrahepatic levels of these compounds. Thus, these receptors regulate the metabolism and excretion of these compounds. Note that a relatively high redundancy exists for several substance classes to bind to multiple receptors.