Butyrylcholinesterase (EC 3.1.1.8; BChE) is a sister enzyme of acetylcholinesterase. Though BChE lacks obvious physiological functions, it is of toxicological and pharmacological importance in detoxifying or catabolising ester-containing drugs. Furthermore, individuals deficient in BChE appear asymptomatic, apart from a heightened sensitivity to the muscle relaxants suxamethonium and mivacurium, two BChE substrates used as myorelaxant. Although many acquired conditions may affect BChE activity, BChE deficiency is mainly due to mutations in the BCHE gene (OMIM 177400). Currently, more than 70 natural mutations have been documented in human BCHE. They have an adverse effect on BChE activity by affecting the catalytic functioning or the protein expression. However, the atypical variant (rs1799807) is the most frequently involved in prolonged apnea.