It is found in a non coding regulatory region (a "switch") upstream of the lactase gene.
The primary carbohydrate in milk, lactose, is an important source of energy for most young mammals.
Lactase is the enzyme that hydrolyzes lactose into glucose and galactose (or lactase-phlorizin-hydrolase, LPH). Since milk is a necessary part of young mammals' diets and since mammals cannot utilise lactose without this enzyme, lactase activity is crucial to the early development of most mammals.
Although the methods and evolutionary justifications for this downregulation are not well understood, lactase synthesis often falls once the weaning period is over. However, some people continue to produce lactase far into adulthood and can metabolize the lactose in fresh milk. This quality is known as LP (Lactase persistence).
Learn more about Lactase persistence here:
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