Two ultraviolet-sensitive cell lines, designated S-1M and S-2M, were isolated from the original HeLa S3 cells by the N-methyl-N′-nitro-N-nitrosoguanidine (MNNG)-ultraviolet light (UV) and 5-bromodeoxyuridine (BUdR)-visible light method. The D0 values were 87, 68, and$45\ {\rm ergs}/{\rm mm}^{2}$, and the extrapolation numbers (n) were 1.4, 1.0, and 1.0 for HeLa S3, S-1M, and S-2M cells, respectively. The average initial excision rate of UV-induced thymine dimers in the UV-sensitive cells is approximately one fifth that in HeLa S3 cells. The UV-sensitive cells may appear in the original HeLa S3 population due to the mutagenic action of MNNG. S-2M cells are more sensitive to the carcinogen 4-nitroquinoline 1-oxide (4-NQO) than HeLa S3 cells. However, there are no significant differences in sensitivity to x-irradiation among the three cell lines. The findings indicate that the 4-NQO-induced DNA damage and its repair are similar to that found after UV-irradiation, but different from that found after x-irradiation.

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