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Deficiency of the brush border enzyme lactase-phlorizin hydrolase (LPH), which catalyzes the hydrolysis of lactose to yield glucose and galactose, leads to malabsorption of lactose and to lactose intolerance when unabsorbed lactose reaches the colon and is fermented by bacteria (1, 2). Adult hypolactasia, which reflects declining LPH gene expression (3-5) during maturation, varies in frequency among populations (6). A single-nucleotide polymorphism (SNP) upstream of the LPH gene at position -13910 (C [right arrow] [T.sub.-13910]) was reported to perfectly match with phenotypic hypolactasia and lactose malabsorption for the C/C genotype (7). Further studies demonstrated that the C/C genotype matched with low LPH-specific mRNA expression and low lactase activity in intestinal biopsies, suggesting primary adult hypolactasia, whereas the genotypes C/T and T/T matched with high LPH-specific mRNA expression and high lactase activity, strongly suggesting lactase persistence (7-11). A second described SNP, LPH G [right arrow] [A.sub.22018]. is thought to be only in linkage disequilibrium with the SNP C [right arrow] [T.sub.-13910] (11).