 cancer is a worldwide public health concern . 
 identifying carcinogens and limiting their exposure is one approach to the problem of reducing risk 
 . currently , epidemiology and rodent bioassays are the means by which putative human carcinogens are identified . 
 both methods have intrinsic limitations : they are slow and expensive processes with many uncertainties . the development of methods to modify specific genes in the mammalian genome 
 has provided promising new tools for identifying carcinogens and characterizing risk . 
 transgenic mice may provide advantages in shortening the time required for bioassays and improving the accuracy of carcinogen identification ; transgenic mice might now be included in the testing armamentarium without abandoning the two - year bioassay , the current standard . 
 we show that mutagenic carcinogens can be identified with increased sensitivity and specificity using hemizygous p53 mice in which one allele of the p53 gene has been inactivated . 
 furthermore , the tg.ac transgenic model , carrying a v - ha - ras construct , has developed papillomas and malignant tumors in response to a number of mutagenic and nonmutagenic carcinogens and tumor promoters , but not to noncarcinogens . 
 we present a decision - tree approach that permits , at modest extra cost , the testing of more chemicals with improved ability to extrapolate from rodents to humans.imagesfigure 1.figure 2.figure 3 . 
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