Tumor Induction in Mice by Radioactive Thymidine

thymidine in mice and to investigate the possibility of inducing tumors with tritiated thymidine. Although it appeared likely on theoretical grounds that tritiated thymidine could be carcinogenic under appropriate experimental conditions, there were no reports to that effect in the literature at that time, and none have appeared since then with the exception of our own earlier communications. Thymidine, as a precursor of DNA, is incorporated almost exclusively into the nucleus of cells synthesizing DNA prior to mitosis (3). About 10 % of the injected thymidine is incorporated into new DNA of proliferating cells (4), while the remainder is promptly catabolized to tritiated water and other nonvolatile compounds (5). The biological half-life of tritiated water in mice is from 24 to 36 hours (6, 7), while the thymidine incorporated into DNA remains there until the death of the cell or its descendants. The 3-particles from tritium incorporated into DNA from H3-thymidine produce an irradiation which primarily affects the cell nucleus, since about 90% of the energy (0.01795 Mev) of these particles is dissipated within the 1 This work was supported in part by contract AT(11-1)-1131 from the U. S. Atomic Energy Commission, and in part performed under the auspices of this commission.