The ALDH2 gene maps to chromosome 12q24.2. A single nucleotide polymorphism (SNP) at exon 12 predicts lysine at residue 504 instead of glutamic acid. The common form of the SNP (rs671) (504glu) encodes the glu (G) allele and was previously referred to as the ALDH2 *1 allele; the 504lys (A, formerly ALDH2 *2 and 487lys) allele produces a catalytically inactive isozyme (Yoshida et al. 1991), which has greatly reduced or no ability to metabolize acetaldehyde (Yoshida et al. 1984). Individuals homozygous for the lys allele (lower frequency in alcoholic subjects) therefore have greatly reduced capacity to clear acetaldehyde and typically show facial flushing and nausea after alcohol consumption, whereas heterozygotes exhibit similar but less severe reactions (Mizoi et al. 1994). This variant can partially determine blood acetaldehyde concentrations after drinking: heterozygotes or homozygotes for the 504lys allele showed peak blood acetaldehyde concentrations after alcohol consumption 6- and 19-fold higher than homozygous common-allele individuals, respectively (Mizoi et al. 1994). Due to delayed oxidation in individuals with the 504lys allele, these individuals have high blood acetaldehyde concentrations, which can cause adverse reactions
