Casein sodium salt from bovine milk is a phosphoprotein. There are four main types of Casein which make up approximately 80% of the total protein in bovine milk: a-s1 Casein, a-s2 Casein, ß-Casein, and ?-Casein. Casein is proposed to be the main protective constituent in milk. Casein protein has numerous experimental applications including use as a blocking agent in immunochemistry, recovery of enzyme activity from SDS extracted samples, and as a substrate for protease and kinase assays. a-Casein can be an immunogenic in young children, and anti-a-casein antibodies have strong cross-reactivity with insulin. This effect can explain the uncommon insulin autoimmunity, but it does not appear to progress to Type I diabetes.