The reduction in frequency of X-ray-induced sister chromatid exchanges in cultured mammalian cells during post-irradiation incubation in Hanks' Balanced Salt Solution

The frequency of sister chromatid exchanges (SCE) had an exponential dependence on dose in X-irradiated HeLa cells. When the cells were held in Hanks' solution the frequency of SCE decreased to approximately 40% of that immediately after irradiation at all doses. The decrease took place within one hour of the post-irradiation treatment while the increase in colony-forming ability (potentially lethal damage repair) induced by the treatment required more than 8 hours to reach a maximum. The frequency of SCE after 1000 R of X-irradiation decreased from 5.1 to 2.7, but did not reach the spontaneous level, 1.0, even after 24 hours. The operation of two different repair processes is indicated. (U.K.)