The magnitude of mitotic delay and the incidence of chromosomal aberrations depend upon the stage at which irradiation occurs in the cell generation cycle of synchronized Chinese hamster cells in vitro. Because cysteamine has a differential protective effect upon cell lethality during the cell cycle, the dependence of its protective effect against mitotic delay and chromosomal aberrations upon cell cycle position was investigated. The results show that cysteamine-treated cells have a pattern for radiation-induced mitotic delay and chromosomal aberrations similar to that for untreated cells, except that the dose required is much higher. The fact that a differential effect was not observed may be because the degree of synchrony was not sufficient to permit the detection of small differences. However, the magnitude of the dose-modifying factor for cysteamine differed in each assay from those previously reported for lethality. The dose-modifying factor for mitotic delay was about 2.5 throughout the cell cycle, for single-hit aberrations it was about 8, and for two-hit aberrations, about 5.

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