Carboxylesterase 2 (CES2) is a member of the carboxylesterase family and belongs to the multigene family. Carboxylesterase 2 is responsible for the hydrolysis of ester- and amide-bond-containing drugs such as cocaine and beroin. It also serves to hydrolyze long-chain fatty acid esters and thioesters. It is speculated that carboxylesterases may play a role in lipid metabolism and the blood-brain barrier system and together with isform 1, are a serine esterase involved in both drug metabolism and activation. Human carboxylesterase 2 is commonly expressed in tumor tissues and irinotecan, a topoisomerase I inhibitor commonly used in the treatment of many solid tumors.