Expression of lethal mutations in progeny of irradiated mammalian cells

The cloning efficiency of the progeny of CHO and BALB/3T3 cells surviving acute exposures of 100-1000 cGy of X-rays was examined by three different experimental protocols. A dose-dependent decrease in cloning efficiency was observed with both cell types up to 13-15 population doublings after exposure. This decrease persisted for longer times after high radiation doses; for example, the cloning efficiency of the progeny of 3T3 cells 28 population doublings after irradiation with 1000 cGy was 44 per cent of that in parallel non-irradiated controls. Confluent holding under conditions which allowed the repair of potentially lethal damage had no effect on this phenomenon. These results are consistent with the hypothesis that large numbers of lethal mutations may be expressed among the progeny of surviving cells for many generations after irradiation.