Deficiencies for b-casein and a-casein exist as naturally occurring genotypes in goats [17] but have no apparent detrimental effect on milk production. However, the aS1-Cn0 allele has been reported to decrease the efficiency with which other casein proteins are secreted [18] . We set out to define the role of acasein in milk secretion and its nutritional role by inactivating the corresponding gene in mice. Our results demonstrate that a-casein deficiency has a significant effect on milk protein secretion and growth of the offspring.