URL of this page: https://medlineplus.gov/genetics/gene/si/

SI gene


Normal Function

The SI gene provides instructions for producing the enzyme sucrase-isomaltase. This enzyme is found in the intestinal tract, where it is involved in breaking down the sugars sucrose (a sugar found in fruits, and also known as table sugar) and maltose (the sugar found in grains). Sucrose and maltose are called disaccharides because they are each made up of two simple sugar molecules. Disaccharides must be broken down into simple sugar molecules to be digested properly.

The sucrase-isomaltase enzyme is found on the surface of the intestinal epithelial cells, which are cells that line the walls of the intestine. These cells have fingerlike projections called microvilli that absorb nutrients from food as it passes through the intestine. Based on their appearance, groups of these microvilli are known collectively as the brush border. The role of the sucrase-isomaltase enzyme is to break down sucrose and maltose into simple sugars so that they can be absorbed by microvilli into intestinal epithelial cells.

Health Conditions Related to Genetic Changes

Congenital sucrase-isomaltase deficiency

At least 10 mutations in the SI gene have been found to cause congenital sucrase-isomaltase deficiency. These mutations disrupt the folding and processing of the sucrose-isomaltase enzyme, transportation of the enzyme within the intestinal epithelial cells, the orientation of the enzyme to the cell surface, or its normal functioning. An impairment in any of these cell processes results in a sucrase-isomaltase enzyme that cannot effectively break down sucrose, maltose, or other compounds made from these sugar molecules (carbohydrates). The inability to digest these sugars causes the intestinal discomfort seen in people with congenital sucrase-isomaltase deficiency.

More About This Health Condition

Other Names for This Gene

  • MGC131621
  • MGC131622
  • Oligosaccharide alpha-1,6-glucosidase
  • sucrase-isomaltase (alpha-glucosidase)

Additional Information & Resources

Tests Listed in the Genetic Testing Registry

Scientific Articles on PubMed

Catalog of Genes and Diseases from OMIM

Gene and Variant Databases


  • Jacob R, Zimmer KP, Schmitz J, Naim HY. Congenital sucrase-isomaltase deficiency arising from cleavage and secretion of a mutant form of the enzyme. J Clin Invest. 2000 Jul;106(2):281-7. doi: 10.1172/JCI9677. Citation on PubMed or Free article on PubMed Central
  • Keiser M, Alfalah M, Propsting MJ, Castelletti D, Naim HY. Altered folding, turnover, and polarized sorting act in concert to define a novel pathomechanism of congenital sucrase-isomaltase deficiency. J Biol Chem. 2006 May 19;281(20):14393-9. doi: 10.1074/jbc.M513631200. Epub 2006 Mar 16. Citation on PubMed
  • Ritz V, Alfalah M, Zimmer KP, Schmitz J, Jacob R, Naim HY. Congenital sucrase-isomaltase deficiency because of an accumulation of the mutant enzyme in the endoplasmic reticulum. Gastroenterology. 2003 Dec;125(6):1678-85. doi: 10.1053/j.gastro.2003.09.022. Citation on PubMed
  • Sander P, Alfalah M, Keiser M, Korponay-Szabo I, Kovacs JB, Leeb T, Naim HY. Novel mutations in the human sucrase-isomaltase gene (SI) that cause congenital carbohydrate malabsorption. Hum Mutat. 2006 Jan;27(1):119. doi: 10.1002/humu.9392. Citation on PubMed

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