The most critical consequence of a-casein deficiency for the offspring is a sustained shortfall in body size. Pups nursed by acasein deficient dams only reach 25% of body weight of control animals at weaning and even at later stages in life remain around 33% lighter than control animals. The experiments described in this paper have been carried out with 3 litters of each genotype on a C57BL/6 background. However we have observed the same phenotype of reduced weight gain during lactation with two more litters on a CD1 background (Kolb et al., unpublished data) , suggesting that the phenotype is similar in different genetic backgrounds. The observed changes in calcium concentration in the milk of a-casein deficient animals are unlikely to play a role in this permanent reduction in body size as calcium restriction during lactation only exerts a transient growth delay in rodents [42] .