The translation of polyadenylated and of non-polyadenylated RNA obtained from 
lactating rat mammary gland was almost totally inhibited by 0.5 mM 
7-methylguanosine-5'-phosphate in the wheat-germ cell-free system. This 
inhibition was maintained during the preparation of the 9S whey-protein mRNA and 
of the 12S and 15S casein mRNAs. Chemical decapping of these mRNAs caused a 
similar reduction of their activity. Although a large fraction of milk-protein 
mRNAs have been reported to lack 3'-polyadenylation, these results show that the 
mRNAs in the mammary gland do contain a 5'-terminal 7-methylguanosine cap.