Abstract
In leukemic cells cultivated with ribonucleic acid (RNA) obtained from the spleen of calf embryo, the appearance of tryptophan-pyrrolase has been demonstrated. This enzyme has not been observed in the cell stock used for the experiment, while it is present in the splenic tissue of the calf embryo. The addition of 5-azauridine and uridine to the culture mixture showed the appearance of the enzyme in the leukemic cells to be due to a direct influence on the deoxyribonucleic acid (DNA) of the cells themselves.