Milk proteins are generally composed of proteins such as casein, lactalbumin, lactoglobulin, blood serum albumin, and immunoglobulin. One of them, casein from milk proteins consists of fractions such as alpha, beta, kappa and gamma. Beta casein gene in cattle consists of 209 amino acids and contains proline amino acid (CCT) at 67th position of the amino acid sequence. The milk obtained from animals with this genotype is defined as the original A2 milk. Proline/Histidine amino acid substitution (C/A) at 67th position of the amino acid sequence results in the beta-casomorphin-7 (BCM-7) which is a naturally occurring opioid peptide from cow’s milk. By this substitution (CAT), the milk obtained from animals with genotypes (A1A1 or A1A2) is defined as the A1 milk. BCM-7 is a polypeptide consisting of 7 amino acids and is produced from A1 milk during digestion of beta-casein protein. As a result, BCM-7 has harmful effects on human health. The aim of this study is to raise awareness in the field of animal science as well as to contribute to the taking of professional measures by using the literature on A1 / A2 milk and beta-casomorphin-7.