To date, no other gene has been identified that harbors variations with effects on alcohol dependence as strong as those of the ALDH2*2 or ADH1B*2 alleles. Other alleles of the ADH and ALDH genes also have been reported to affect risk; however, these effects are much smaller and are not detected in all studies. For example, several studies found other variations in and near the ADH1B gene, as well as in or near the ADH4, ADH1C, ADH5, ADH6, and ADH7 genes that affect risk for alcoholism or the level of alcohol consumption (see the article by Hurley and Edenberg, pp. 339–344). Importantly, many of these other alleles do not affect the structure of the encoded protein but probably act by altering the level of gene expression. Therefore, it is important to also study the effects of various alleles on gene regulation. Another complication is that many ADH and ALDH genes are located on the chromosomes in clusters, and many nearby variations therefore are inherited together (i.e., in haplotype blocks). As a result, researchers cannot always determine which allele in such
