The MCM6 gene is involved in making lactase.
Genetic variations in this gene are associated with adult hypolactasia (lactose intolerance). Lactase is the enzyme that digests the lactose in milk, and while there is plenty of it around in mammalian infants, including most humans, lactase activity generally declines after weaning.
In some individuals, though, lactase activity persists at a high level throughout adult life, enabling them to digest lactose as adults. This dominantly inherited genetic trait is known as lactase persistence; the more evolutionarily "normal" lowered level of lactase can lead to the inability to digest lactose as an adult and thus lactose (and milk) intolerance.
Lactose intolerance typically increases with age, and a reasonable percentage of individuals with symptoms of irritable bowel syndrome may actually be lactose intolerant.