Karyotypic abnormality of the X chromosome is rare in mutant HPRT^- lymphocyte clones

Lymphocyte clones mutated at the hypoxanthine-guanine phosphoribosyl-transferase (HPRT) locus on the X chromosome were studied by synchronization and G banding to determine the proportion of mutant clones having visible karyotypic change. 47 spontaneously mutant clones, 17 mutant clones induced by X-irradiation and 33 wild-type clones were studied. All clones were karyotypically normal except for 1 clone induced by X-irradiation in which an interstitial deletion of the short arm of the X chromosome had been inserted into the long arm of the same chromosome between q23 and q24; this change may have been coincidental or may have resulted in a position effect mutation. It was concluded that the great majority of mutations were not associated with a visible chromosome abnormality. This conclusion complements molecular studies which suggest that gene changes at the HPRT locus in HPRT^- mutants generally extend over segments of DNA too small to be resolved by karyotypic analysis.