The transgenic cell lines G12 and G10, each with tants had deleted the transgene, whereas the delea bacterial gpt gene stably integrated at a single tion mutant frequency was increased to about 50% but different position in the Chinese hamster ge-of the X-ray-and bleomycin-induced G12 mutants. nome, were evaluated for deletion of the gpt In contrast, both spontaneous and induced deletion transgene following exposures to several clasto-frequencies are considerably higher for the G10 gens. More than 150 independently cloned G12 cell line. Among spontaneous G10 mutants, up to and G10 6-thioguanine-resistant mutants have been 50% have deleted the gpt transgene, whereas alcharacterized by polymerase chain reaction (PCR) most all of the X-ray-and bleomycin-induced G10 amplification and Southern blots in this study. De-mutants have lost the integrated gene sequence. spite differences in the integration sites for the gpt These results are discussed in the context of the gene in the G12 and G10 cells, PCR amplification transgene integration sites and the influences of the of the gpt gene from both cell lines can be per-surrounding genome that may render certain geformed using the same single set of primers. By PCR netic regions prone to deletion. Environ. Mol. Mutadeletion screening, about 20% of recovered sponta-gen. 30:418-428, 1997 แญง 1997 Wiley-Liss, Inc. neous 6-thioguanine resistant (6TG R ) gpt 0 G12 mu-