This review, written for non-specialists, describes briefly the steps of protein biosynthesis from their precursors in the blood to the excreted molecules, taking rat gamma-casein as an example. A schematic description is given of the procedures employed for preparing cDNAs which can provide the sequences of the corresponding mRNAs and proteins. Recent findings concerning milk proteins are briefly mentioned, and in particular those dealing with sequence determinations of mRNAs coding for milk proteins from several species.