Carboxylesterases are enzymes that metabolize a growing number of commonly used medications. In humans, two carboxylesterases, carboxylesterase-1 (hCE1) and carboxylesterase-2 (hCE2), found primarily in the liver and intestine respectively, play an important role in the biotransformation of numerous classes of commonly used drugs containing ester groups including ACE inhibitors, anticancer agents, opiate analgesics, HMG-CoA reductase inhibitors (statins), CNS stimulants, antiviral medications, and antiplatelet agents.