Casein is one of the major proteins in milk. Originally cows made casein called A2. Then 1000's of years ago a mutation happened and some cows started making a slightly different type of casein called A1. Most modern milk has both A1 and A2 casein proteins. Studies show that milk with only A2 casein may be easier for some people to digest than milk that contains all or some A1. So how do you get milk with just good old fashioned A2 protein? Identify cows that make just A2 casein, gather them together and milk them!