Genomic location and gene structure of BChE and AChE. Both enzymes encode many different variants, BChE due to multiple polymorphisms and AChE because of alternate promoter usage and 3’ alternative splicing. Noted are those single nucleotide polymorphisms in BChE which generate the two most frequent variants in Middle Eastern populations, the C-terminal A539T substitution in BChE-K and the N-terminal D70G mutation in “atypical” BChE which causes post-anesthetic depression and is linked to apnea.⁴³ BChE-K retains approximately 70% of its hydrolytic activity due to inherent instability, whereas “atypical” BChE retains only 25% of this activity. In AChE, alternate promoters generate N-terminally extended variants, and alternative splicing changes the C-terminus of the protein.³⁹