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Schematic of radiation-induced mutagenesis. Open circles represent normal wild-type cells, while closed circles represent mutated cells. B is an example of a cell directly mutated by radiation exposure; the mutation is transmitted to all of its progeny. However, most of the cells in the irradiated population will retain the wild-type phenotype (A). C and D are examples of mutations arising as a result of radiation-induced genomic instability. The irradiated cell and its immediate progeny are wild type, but the frequency with which mutations arise amongst the more distant descendants of the irradiated cell is elevated.