CLD is caused by a deficiency of intestinal lactase activity (EC 3.2.1.23) which is the result of mutations of the lactase-phlorizin hydrolase gene (LCT) whose 49.3 kb of genomic DNA organized in 17 exons are located on 2q21.3 [3–5]. Here, LCT is linked to congenital secretory chloride diarrhea 1.