A2 milk refers to the milk that contains mostly A2 β – casein protein. A2 milk is found basically in indigenous cows and buffaloes of India. 

Milk proteins are generally composed of proteins such as casein, lactalbumin, lactoglobulin, blood serum albumin, and immunoglobulin. Casein is the chief component of the milk proteins and consists of fractions such as alpha, beta, kappa and gamma proteins of which about 30 percent is beta- casein. The major beta- casein variants are A1 and A2. Beta- casein consist a chain of 229 amino acids. Cows that produce milk contain proline amino acid are called as A2 cows. A2 milk is found basically in indigenous cows and buffaloes of India. Around 5,000 years ago, a mutation occurred in this proline amino acid, converting it to histidine amino acid at 67 th position. Cows that have this mutated beta casein protein are called A1 cows. Different mutations bovine beta casein produce 13 genetic variants and out of these A1 and A2 are the most common. A1 β casein is enzymatically broken down in the intestine to produce beta – casmorphin- 7 (β-CM-7) which is an opioid peptide similar to morphine. BCM-7 is toxic in nature which interacts with the human gastrointestinal tract, internal organs and brainstem.