Inactivation of the b-casein gene has shown to have little effect on milk secretion and growth of the offspring in the mouse [5] . In contrast, deletion of the k-casein gene in mice completely abrogates milk production [6] . Exactly why this happens is unclear as neither the structure of the individual casein proteins nor the structure of the casein micelles is currently known in detail [7, 8, 9] . The available results, however, support a model in which k-casein (the casein protein which is present in milk in least abundance) plays a critical role in ensuring transport and solubility of casein micelles [7] . The sequences of a-(aS1 in ruminants) b-, and ccasein (aS2-casein in ruminants) contain one or more clusters of phosphorylated residues known as phosphate centres. Phosphate centres can sequester amorphous calcium phosphate, probably in the Golgi vesicles of mammary secretory cells, to form thermodynamically stable complexes of defined chemical composition which are then secreted through the apical membrane of the mammary epithelial cells [10, 11, 12, 13, 14] . Milk, like many other biological fluids, is supersaturated with respect to the crystalline mineral of bones and teeth (apatite) but due to the sequestration reaction, is under-saturated with respect to amorphous calcium phosphate. Since apatite only forms by maturation of the amorphous phase of calcium phosphate, it cannot form at all from milk and this helps to protect the mammary gland against soft tissue mineralization [15] . In most milks, the majority of the total calcium is sequestered within the casein micelles. In milks of different species, the total calcium and total casein concentrations are highly correlated [16] and provided this balance is maintained, the milks remain stable. If there is insufficient casein to sequester the secreted calcium and orthophosphate then the milk becomes unstable [15] . In mouse milk, 3 of the phosphate centres are in present a-casein and only one in b-casein. Thus the loss of a-casein is likely to have more serious consequences for milk stability than the loss of b-casein.