Fluctuation analyses of spontaneous mutations to 6-thioguanine resistance in Chinese hamster ovary cells in culture

Fluctuation analyses of the spontaneous appearance of 6-thioguanine (TG)-resistant mutants in cultured Chinese hamster ovary (CHO) cells were performed to investigate (1) whether the resistance is induced by the selective agent or is the result of a mutation which occurs prior to the TG selection and (2) to estimate the spontaneous mutation rate at the hypoxanthine—guanine phosphoribosyl transferase (hgprt) locus. The potential problem of phenotypic delay was minimized by allowing an adequate expression time through maintenance of the cultures in a division-arrested, viable state. The results demonstrate that the TG-resistant (TG^r) cells arise randomly in the cultures, independently of the selective agent, which is consistent with spontaneous mutations. The average values for mutation rate ± standard deviation, based on 4 independent determinations and 2 methods of calculation, are 3.4 ± 1.2 × 10^?7 (median method) and 5.1 ± 1.8 × 10^?7 (mean method) mutants/cell/generation.