Cows with the A2A2 gene produce only A2 milk. While all cows produce at least some A2 beta-casein protein, certain breeds have predominantly A2 in their milk. These breeds include Guernsey, Jersey, Charolais, and Limousin cows. Holstein, Friesian, Ayrshire, and British Shorthorn cows produce milk with roughly equal amounts of A1 and A2 beta-casein protein. Therefore, farmers can determine the status of their herd by genomic testing their cows and heifers. They can then practice selective breeding to produce an A2 herd, which can take many generations depending on the status of the herd. Some farmers have transitioned to A2 herds, and many A.I. studs have been marketing A2A2 bulls. However, claims that A2 milk is easier for humans to digest and improves health have not been proven by science.