mammalian cells in culture have been used to study the genetic effects of physical and chemical agents . we have used chinese hamster ovary ( cho ) cells , clone k1-bh4 , to quantify mutations at the x - linked , large ( 35 kb ) hypoxanthine - guanine phosphoribosyltransferase ( hprt ) locus ( the cho / hprt assay ) induced by environmental agents . by transfecting an hprt - deletion mutant cho cell line with the plasmid vector psv2gpt , we isolated a transformant , as52 . 
 as52 cells carry a single functional copy of an autosomal , small ( 456 bp ) xanthine - guanine phosphoribosyltransferase ( gpt ) gene ( the bacterial equivalent of the mammalian hprt gene ; as52/gpt assay ) . 
 we found that ionizing radiations such as x - rays and neutrons and oxidative genotoxic chemicals such as adriamycin , bleomycin , hydrogen peroxide , and potassium superoxide are much more mutagenic to the gpt gene in as52 cells than to the hprt locus in k1-bh4 cells . 
 the hypermutability of the gpt gene probably results from a higher recovery of multilocus deletion mutants in as52 cells than in k1-bh4 cells , rather than a higher yield of induced mutants . 
 these results demonstrate that the use of the hprt locus alone could lead to an underestimate of the genetic risk of these agents . 
 analyses of the mutation spectrum using a polymerase chain reaction - based deletion screening and dna sequencing procedure showed that a high proportion of hprt- and gpt- mutants induced by x - rays carry deletion mutations . 
 thus , both the mutant frequency and mutation spectrum need to be considered in assessing the genetic risk of ionizing radiation and oxidative genotoxic chemicals.imagesfigure 2 . 
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