Lactase persistence is the continued activity of the lactase enzyme in adulthood. These enzymes include peptidase, sucrase, maltase, lactase and intestinal lipase. Primary lactose intolerance occurs as the amount of lactase declines as people age. Primary lactose intolerance is when the amount of lactase decline as people age. Examples are lactase, alcohol dehydrogenase and DNA polymerase. Hypolactasia (lactase deficiency) is distinguished from defect. People with primary lactase deficiency cannot modify their body's ability to produce lactase. Lactase persistence was determined with hydrogen breath tests. Some populations have developed genetic changes that resulted in ongoing production of lactase. Hence, it was in these regions that genes for lifelong lactase production first evolved. This genetic defect is characterized by a complete lack of lactase (alactasia). The genetic variations for lactase persistence and greater height came with the Yamnaya people. This makes the study of lactase persistence distribution more difficult. Lactase persistence-frequencies are highly variable. Despite their pastoral lifestyle, there was little evidence of lactase persistence.