In spite of the fact that the deuterium concentration is over 10 mmol/l in all living organisms, its possible role has been ignored for six decades. Recent studies have shown that the depletion of the naturally occurring deuterium can result in tumour regression in mice, dogs, cats and humans. The effect of deuterium depletion on gene expression plays a key part in tumour development. The carcinogen, 7,12-dimethylbenz(alpha)anthracene (DMBA), was used to increase gene expression in "short term" investigations. The expression of c-myc, Ha-ras and p53 gene was followed in CBA/Ca sensitive inbred mice drinking tap water or deuterium-depleted water (DDW) after induction. By detecting the RNA expression 48 hours after exposure to the carcinogen it was found that the expression of all genes investigated was inhibited in six different organs (spleen, lung, thymus, kidney, liver and lymph node) in the DDW-treated group. It is suggested that genes playing a key role in the cell cycle regulation and tumour development are sensitive to deuterium depletion.