AFTER the discovery of autosomal trisomies in man, the activity of various enzymes in trisomic cells was measured in the belief that proportionality between gene dose and enzyme activity would help in identifying genes carried by trisomic chromosomes1–5. It became apparent, however, that the trisomic state could produce changes in enzyme activity unrelated to gene dose, probably by interfering with normal physiological processes6. Therefore, verification of the concept that a simple rapport of proportionality between gene dose and concentration of secondary gene product could exist in cells with auiosomal trisomy, as it does in diploid cells with gene mutations or in mouse eggs with different numbers of X chromosomes7, had to await the recent advances in human chromosome mapping. We report here the results of a study of gene dose effects in trisomy 16.