It is commonly believed that all proteins in our gastrointestinal tract will simply digest into amino acids that get into blood and lymph and will be part of our proteins. But this is not quite so: it has been shown that two-, three- and oligopeptides can also be absorbed. One of the milk proteins, β-casein, can be a source of one of the oligopeptides, β-casomorphin. He was nicknamed “the devil in milk,” because epidemiological studies have shown that the consumption of cow’s milk correlates with the probability of the development of a newborn’s sudden death syndrome, diabetes, obesity in children and heart pathologies. The mechanisms of development of these pathologies are based on the ability of β-casomorphin to activate the μ1 morphine receptor. But β-casomorphin can only be formed from milk of a certain type – it was named A1. Replacing a single nucleotide in a gene encoding β-casein leads to the fact that this oligopeptide is not formed in the gastrointestinal tract, as there is an additional site for the action of digestive enzymes. Milk with such β-casein called A2.