The average desoxypentosenucleic acid content of individual nuclei was determined for various normal and tumor tissues, and for livers showing precancerous changes, in the rat. With certain exceptions attributable to polyploidy, the values were practically indistinguishable from each other and from values reported for cell nuclei of other mammals. The amount of this nucleic acid in diploid cells of the rat appears to be a constant, nearly equal to 6 x 10–6 micrograms. Findings of increased concentrations of this nucleic acid in tissues showing preneoplastic or neoplastic changes therefore confirm histological observations of increased cellularity and polyploidy in such tissues.

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