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SI gene


Normal Function

The SI gene provides instructions for producing the enzyme sucrase-isomaltase. This enzyme is made in the cells that line the small intestine, 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). This enzyme is also important for digesting starches, which are first broken down into sucrose and maltose. 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 used by the body. 

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

Many variants (also known as mutations) in the SI gene have been found to cause congenital sucrase-isomaltase deficiency.  These variants disrupt the folding and processing of the sucrase-isomaltase enzyme, transportation of the enzyme within the intestinal epithelial cells, localization of the enzyme at the correct 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 sugars (carbohydrates) from starchy food. Rather than being absorbed by the small intestine, the undigested sugars move to the large intestine (colon). Here, they attract water and are consumed by normal bacteria in the colon, causing intestinal discomfort in people with congenital sucrase-isomaltase deficiency.

More About This Health Condition

Other Names for This Gene

  • MGC131621
  • MGC131622
  • 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


  • Husein DM, Wanes D, Marten LM, Zimmer KP, Naim HY. Heterozygotes Are a Potential New Entity among Homozygotes and Compound Heterozygotes in Congenital Sucrase-Isomaltase Deficiency. Nutrients. 2019 Sep 25;11(10):2290. doi: 10.3390/nu11102290. Citation on PubMed
  • 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

The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.