Caseins are a major group of secretory phosphoproteins synthesized during lactation in mammals and are stored and secreted as stable calcium phosphate complexes called micelles. The micelles are composed of AlphaS1, AlphaS2 and Beta caseins which interact with calcium and Kappa-casein. Kappa-casein has two important functions in the lactational process of mammals, one to stabilize milk micelles which can be assimilated slowly and second it has a labile band in its primary structure which is important for milk clotting. Therefore, production of Kappa-casein represents an important step in the functional differentiation of the mammary epithelium and an alteration of this production may be a marker of neoplastic transformation. A full length cDNA clone for the rat Kappa-casein was isolated and its nucleotide sequence was determined. The deduced amino acid sequence from the nucleotide sequence revealed a signal peptide of 21 amino acids and a mature protein of 203 amino acids long. The mature protein is 33 amino acids long at the carboxyterminal end as compared to the known Kappa-caseins. Kappa-casein mRNA content of the mammary tissue was found to increase during its functional differentiation. Prolactin appears to modulate the production of Kappa-casein mRNA both in normal mammary cell and some carcinogen induced mammary tumors.