The CYP2A6 gene is one of the three members of the human CYP2A gene subfamily, the others being CYP2A7 and CYP2A13. The CYP2A6 enzyme catalyses the oxidation of several compounds that have clinical or toxicological interest, including pharmaceuticals, procarcinogens, and tobacco smoke constituents. CYP2A6 is expressed mainly in liver, and only trace amounts are found in extrahepatic tissues. Coumarin is a high-affinity substrate for CYP2A6, and a phenotyping test based on coumarin 7-hydroxylation has been developed. Two mutant alleles of the CYP2A6 gene have been found, i.e. CYP2A6*2 and CYP2A6*3. Homozygosity for both mutated alleles appears to confer a poor metabolizer (PM) phenotype, detectable by slow or non-existent 7-hydroxylation of coumarin. Very little is known about the inducibility and regulation of CYP2A6, but studies on the mouse orthologue, CYP2A5, have revealed novel pathways for induction. Since CYP2A6 polymorphism was found fairly recently, nothing is known presently about associations between variant CYP2A6 alleles and diseases or other adverse outcomes of exposure to toxins. Such studies, however, are clearly warranted, given the wide range of procarcinogens and other toxins metabolized by the CYP2A6 enzyme.