We analysed the expression of casein specific mRNAs in the lactating mammary gland from control and a-casein deficient mice (Fig. 5b and c) . Expression of the casein genes was correlated with expression of the reference genes GAPDH (shown in Fig. 5b and c) and b-actin (Fig. 5d) . Identical results were obtained for both reference genes. As expected the amount of a-casein specific mRNA in heterozygous mice is at 50% of control levels and absent in homozygous knock-out mice. Expression of the corresponding b-casein and c-casein casein genes is significantly reduced in acasein deficient mice (to around 10% of the expression levels detected in wild-type and heterozygous mice; Fig. 5b, c and d) . This suggests that the absence of the a-casein protein not only interferes with protein secretion from the mammary epithelial cells but that absence of both alleles of the a-casein gene also reduces the expression of the other casein genes. The expression of the kcasein gene is also affected but not to the same extent. Its expression is reduced to around 20% of control levels in a-casein deficient mice (Fig. 5c and d) . No consistent reduction of b-casein, c-casein and k-casein expression is detected in the heterozygous animals. Fig. 5d presents the expression of the casein genes as percentages of wild-type expression. The consistent reduction of b-, cand k-casein gene expression in the a-casein deficient mammary tissue is highly significant (p,0.001; table 6) whereas expression changes in heterozygous vs wild-type mice are marginally or not significant (all p values.0.01; table 6). Histological analysis of mammary tissue in a-casein deficient mice