Abnormally high incidence of SCE in three successive cell cycles in the CHO mutant EM9 as detected by a three-way immunoperoxidase differential staining.

A high resolution three-way immunoperoxidase method for the detection of SCE per cell generation in chromosomes with very low levels of BrdU in DNA has been recently standardized in our laboratory. We have made use of this methodology to assess whether the CHO mutant EM9 is solely hypersensitive to BrdU incorporated into template DNA as regards the extraordinarily high SCE frequency observed as compared with its parental line, AA8, or also shows an elevated spontaneous frequency of exchanges. Our results confirm the importance of BrdU incorporated for SCE in EM9 but also show that the estimated yield of spontaneous SCE is about 10 times higher in this mutant than in the parental line, AA8.