Groups of mice were subjected to X-rays and to neutrons, followed either 1 month or 9 months later with a large dose of CCl4. The animals were sacrificed at 20 months of age, and liver tumors were scored. The CCl4 treatment unmasks the carcinogenic mutations produced by the radiation. It is found that the tumor incidence is about the same for X-rays and neutrons, and that tumor incidence is almost as high when the CCl4 is given 9 months postirradiation as when it is given at 1 month. This indicates that, once a carcinogenic mutation is formed, repair is impossible. This is especially interesting, since chromosomal aberrations scored in regenerating liver cells indicate a marked long-term recovery following X-irradiation but not following neutron irradiation. It is concluded that certain structural repair of chromosomes is possible over protracted periods but repair of functional genetic damage probably is not.

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