In a-casein deficient milk the reduction in protein secretion also affects the mouse WAP protein, which is secreted into the whey fraction of milk (Fig. 2b) . This suggests that all milk proteins secreted from mammary epithelial cells are affected by the absence of a-casein. In contrast, secretion of albumin, which is derived from serum, is not affected by the absence of a-casein (Fig. 2a) . Furthermore, the overall amount of milk secreted by a-casein deficient dams appears to be reduced (observation during harvesting). Thus, a-casein deficiency causes a generalised reduction in milk secretion and a reduction in milk protein concentration.