Milk is the only food for mammals at the early stage of life and serves to ensure all the necessary nutrients and protects the health. Milk is a rich source of proteins, minerals and antibodies. It contains 2 major proteins: casein and whey. Casein is the most important component in milk, accounting for about 80% of all proteins. There are 4 types of caseins found in bovine milk: alpha s1, alpha s2, betta and kappa. Many variants of kappa-casein exist, and the most common are A and B. Variations within the kappa-casein gene result in differences in milk yield, mass fraction of fat and proteins produced by cows. It has been observed that bulls with BB or AB genotype produce heifers with the desirable B variant of kappa-casein gene which is associated with a higher protein yield in milk. In addition, milk containing the BB variant of kappa casein is associated with shorter rennet coagulation, faster and firmer gelling ability and is more suitable for cheese production compared to the AA variant. The E variant has been associated with lower protein content compared to A and B variants. Nowadays genetic tests have increased the efficiency of breeding. Besides phenotypic differences, it is now possible to detect genetic variations that are not visible to the breeders.