Which enzyme deficiency causes lactose intolerance?

Lactase (EC 3.2.1.108) is an enzyme produced by many organisms and is essential to the complete digestion of whole milk. It breaks down the sugar lactose into its component parts, galactose and glucose, simple sugars that can be absorbed into the bloodstream through an animal's intestines. A lactase is a type of β-galactosidase because it breaks down the β-glycosidic bond in D-lactose. The chemical reaction it catalyzes is:C12H22O11 + H2O → C6H12O6 + C6H12O6 + heat. lactose + H2O → β-D-galactose + D-glucose