Lactose intolerance—a person’s inability to break down milk products into sugars—is due to reduced activity of the enzyme lactase. Lactase changes the lactose of mammalian milk into the monosaccharides—glucose and galactose. But if you don’t have enough lactase in your system, it can often lead to mild or severe symptoms from lactose intolerance, which range from mild gastrointestinal discomfort to vomiting and diarrhea. Some genetic connections exist: In particular, congenital lactase deficiency, especially in infants, is a rare, inherited recessive disorder that results in no lactase production and is caused by mutations in what is called the LCT gene. This is in contrast to childhood and adult-onset lactase deficiencies—also inherited and caused by variations in the element that regulates what is called the MCM6 gene.