The chromosome complements of cells of the radioresistant line, which had been derived from HeLa strain by repeated X-irradiation in our previous experiment, were compared with those of the unirradiated original strain of cells. The modal chromosome number of the radioresistant 5 kR line of HeLa cells was found to be 64, and that of the original HeLa strain to be 71. The 5 kR line of cells contained one or two abnormally long acrocentric chromosomes which were almost absent in the parent line. The radiosensitive line of cells from a clone that was obtained by a single cell culture of the original HeLa strain was also examined to confirm the correlation between chromosome constitution and radiosensitivity. From these results and several authors' reports, it was concluded, concerning tumor cells, that the differences in chromosome numbers and karyotypes are simply markers which are useful in distinguishing the resistant line from the original strain.

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