Mutant alleles at the BCHE locus are responsible for suxamethonium sensitivity. Homozygous persons sustain prolonged apnea after administration of the muscle relaxant suxamethonium in connection with surgical anesthesia. The activity of pseudocholinesterase in the serum is low and its substrate behavior is atypical. In the absence of the relaxant, the homozygote is at no known disadvantage. [provided by RefSeq, Jul 2008].