Cancer is due to multiple inherited or acquired mutations occurring in a single cell. DNA replication, although incredibly precise, results in spontaneous errors. A chemical can increase the number of errors per DNA replication by increasing damage to DNA, or it can increase the probability for spontaneous errors by increasing the number of DNA replications. A model has been developed which takes into account the role of DNA reactivity and increased DNA replication in the pluripotential cell population of a tissue on the development of cancer. This model can be used to assess the mode of action of chemicals which are carcinogenic in animal models and to access the relevance of that mode of action to human risk. By using this mode of action-based model, it may then be possible to replace the 2-year cancer bioassay with shorter-term assays which assess DNA reactivity, immunosuppression and estrogenic activity of the chemical and the effects of the chemical on toxicity and cell proliferation.