Genetic variation is one of several factors determining the level and quality of plasma cholinesterase activity (butyrylcholinesterase, BChE). The many genetic variants known today, and the resulting large number of genotypes and phenotypes, have complicated the problem of identifying individual BChE genotypes and phenotypes on the basis of enzymatic analyses alone. Modern molecular biological techniques have, however, permitted the development of diagnostic tests which allow BChE variants to be identified at the DNA level. Today, at least 20 genetic variants have been identified in this way. This review is an introduction to the principles of molecular biology used to identify the variants forms of the BCHE gene.