Neither L cells irradiated with 3 to 20 krad of X rays, nor 8-azaguanine-resistant cells, produced colonies when grown in HAT-selective media. If, however, 106 irradiated cells were mixed with the same number of 8-azaguanine-resistant L cells and uv-inactivated HVJ virus which promotes cell fusion, colonies were always observed. In multiple experiments, between 6 and 53 colonies were counted. Chromosome numbers of two clones isolated following hybridization of 8-azaguanine-resistant cells with L cells irradiated with 5 krad were 68 and 86, whereas chromosome numbers of four clones isolated following 20-krad exposure were 54, 54, 56, and 60. Thus, the higher the X-irradiation dose, the smaller the chromosome numbers in hybrid cells.

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