lactase,enzyme found in thesmall intestine of mammals that catalyzes the breakdown oflactose (milk sugar) into the simple sugarsglucose andgalactose. In humans, lactase is particularly abundant during infancy. It is a so-calledbrush border enzyme, produced by cells known asenterocytes that line the intestinal walls and form the brush border (a chemical barrier through which food must pass to be absorbed). Mutations in thegene that encodes lactase may result in inherited lactase deficiency, whichmanifests aslactose intolerance, or the inability to digest lactose. Lactose that is not absorbed in thegastrointestinal tract undergoes fermentation by bacteria, resulting in the production of gas and intestinal distress.