Acetylcholinesterase, an enzyme essential for the termination of the action of acetylcholine, is encoded by a single gene. Alternative mRNA processing gives rise to the expression of enzyme forms with three distinct carboxyl-termini. These structural differences govern the cellular disposition of the expressed enzyme but do not influence catalytic activity. Alternative polyadenylation signals give rise to distinct 3' non-coding regions which are likely to affect mRNA stability. Alternative splicing also occurs at the 5' end of the gene where two promoter regions can be identified. Hence, regulation of expression of the gene occurs at 3 levels, transcriptional through alternative promoters, translational by affecting mRNA stability and processing of distinct mRNAs and post-translationally by giving rise to distinct peptide chains which are processed differently. Recombinant DNA studies have also been extended to modifying protein structure through site-specific mutagenesis and studying the function of the mutant enzymes.