Mutants resistant to 6-thioguanine were induced in pseudotetraploid hybrid Chinese hamster cells homozygous wild-type at the locus for hypoxanthine phophoribosyl transferase but heterozygous for the linked marker glucose-6-phosphate dehydrogenase. About half of these mutants had concomitantly lost the wild-type allele for glucose-6-phosphate dehydrogenase, as expected if mutation plus chromosome segregation had occurred.