Mouse Carboxylesterases 3 (CES3) is a member of five families of mammalian carboxylesterases that plays a role in catalyzing hydrolytic and transesterification reactions with xenobiotics, anticancer pro-drugs and narcotics, and detoxifying organophosphates and insecticides. Mammalian carboxylesterases are enzymes with broad substrate specificities ranging from small molecule esters to longchain fatty acid esters. It is shown that CESs has key roles in the metabolism of a wide variety of clinical drugs, illicit narcotics and chemical nerve agents. CES3 is broadly expressed in liver, colon and brain.