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SI gene

Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

SI gene

The SI gene produces the enzyme sucrase-isomaltase, which is essential for breaking down dietary sugars like sucrose and maltose in the small intestine.

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 (table sugar) and maltose (found in grains). This enzyme is also important for digesting starches, which are first broken down into sucrose and maltose. These sugars are disaccharides, which 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. These cells have fingerlike projections called microvilli that absorb nutrients from food. 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 (mutations) in the SI gene have been found to cause congenital sucrase-isomaltase deficiency. These variants disrupt the folding, processing, transportation, or localization of the sucrase-isomaltase enzyme at the cell surface. An impairment in these processes results in an enzyme that cannot effectively break down sucrose, maltose, or other carbohydrates.

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.

Other Names for This Gene

  • MGC131621
  • MGC131622
  • sucrase-isomaltase (alpha-glucosidase)
  • SUIS_HUMAN

Frequently Asked Questions

Symptoms usually appear when solid foods containing sucrose or starches are introduced into a baby's diet, often around 6-12 months of age. Common signs include chronic watery diarrhea, abdominal pain, bloating, excessive gas, and failure to thrive due to poor nutrient absorption.
Diagnosis typically involves a hydrogen breath test after consuming a sucrose solution, which measures hydrogen gas produced by bacteria in the large intestine from undigested sugar. A small intestinal biopsy can also be performed to measure sucrase-isomaltase enzyme activity directly. Genetic testing for SI gene mutations can confirm the diagnosis.
The primary treatment is a lifelong diet low in sucrose (table sugar) and starches. This means avoiding many processed foods, sugary drinks, and certain fruits and vegetables. Enzyme replacement therapy, using an oral medication containing sucrase, can also be prescribed to help digest sugars and reduce symptoms.
Congenital sucrase-isomaltase deficiency is a lifelong genetic condition. While the underlying enzyme deficiency does not resolve, symptoms can often be well-managed with strict dietary adherence and, if necessary, enzyme replacement therapy. Some individuals may experience a slight improvement in tolerance to certain sugars as they get older, but dietary vigilance remains important.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.