CRISPR-Cas9-mediated deletion of the LCT intron 2 region in mice results in a 3- to 8-fold reduction in lactase mRNA expression in adulthood, with a greater reduction observed in adults compared to infants, indicating that this conserved genomic region is functionally required for maintaining lactase expression in mature intestinal cells.

In adult intestinal cells, a conserved region inside the lactase gene becomes covered with chemical tags that block its activity, turning off lactase production. This region normally acts like an on-switch that keeps lactase being made after infancy, but as the animal ages, these tags build up and prevent the switch from working. Without this switch, the gene stops making lactase mRNA, so the cell stops producing the enzyme that digests milk.