Lactose intolerance is the inability to digest lactose, due to insufficient amounts of the enzyme lactase. Where is lactase usually found?

The absorptive cells of the small intestine synthesize brush-border enzymes and insert them in the plasma membrane of the microvilli. Thus, some enzymatic digestion occurs at the surface of the absorptive cells that line the villi, rather than in the lumen exclusively, as occurs in other parts of the GI tract. These brush-border enzymes include maltase, sucrase, and lactase, aminopeptidase and dipeptidase, nucleotidases and nucleosidases.
In some people the absorptive cells of the small intestine fail to produce enough lactase. This leads to a condition called lactose intolerance, characterised by diarrhoea, gas, bloating, and abdominal cramps after consumption of milk and milk products.