Carboxylesterases hydrolyze esters of short-chain fatty acids and have roles in animals ranging from signal transduction to xenobiotic detoxification. In enzymology, a carboxylesterase is an enzyme that catalyzes the chemical reaction: a carboxylic ester + H2O = an alcohol + a carboxylate. Most enzymes from this group belong to the superfamily of hydrolases with alpha/beta protein fold (so called Alpha/beta hydrolase fold), specifically those acting on carboxylic ester bonds. The carboxylesterase family of evolutionarily related proteins (those with clear sequence homology to each other) includes a number of proteins with different substrate specificities, such as acetylcholinesterases. Carboxylesterase 3, also known as Liver carboxylesterase 31 homolog and CES3, is a endoplasmic reticulum lumen which belongs to thetype-B carboxylesterase/lipase family. CES3 is involved in the detoxification of xenobiotics and in the activation of ester and amide prodrugs. CES3 shows low catalytic efficiency for hydrolysis of CPT-11, a prodrug for camptothecin used in cancer therapeutics. CES3 is expressed in liver, colon and small intestine.