In molecular biology, the Whey acidic proteins (WAP) have been identified as a major whey protein family in milk, and are important in regulating the proliferation of mammary epithelial cells. Additionally, their physiological function is thought to be similar to a protease inhibitor. It has been concluded, therefore, that WAP regulate the proliferation of mammary epithelial cells by preventing elastase-type serine proteases from carrying out laminin degradation and by suppressing the MAP kinase signal pathway in the cell cycle.