SI and congenital sucrase-isomaltase deficiency: Decreased or absent sucrase and/or isomaltase enzymatic activity has been found in patients with CSID, and investigations at the subcellular and molecular levels in intestinal biopsy specimens have led to the description of several phenotypes, differing in transport efficiency, processing, and sorting of the protein, which result in impaired physiologic functions [17,18,19,20].