With the exception of ADH7, all ADH genes are expressed in adult liver (for a review, see Edenberg 2000). The level of gene expression, however, is strongly influenced by DNA sequences located directly in front (i.e., upstream) of the transcribed DNA region, as well as in introns within the gene and regions downstream of the gene. For all ADH genes, researchers have identified regulatory sequence elements (i.e., promoters) near the transcription start sites (Edenberg 2000). DNA regions that are farther upstream from the genes and which also may contain regulatory elements still need to be examined. Recently, some studies have focused on determining whether different combinations of SNPs (i.e., different haplotypes) in the promoter regions have different effects on gene expression. A single SNP located 136 basepairs (bp) upstream of the translational start site of ADH4 results in a two-fold difference in gene expression in vitro (Edenberg et al. 1999). For the ADH1C gene, expression differs by 40 percent between various haplotypes in a regulatory element located from 3,910 to 3,496 bp upstream of the transcription start site; in addition,
