The mouse model of multi-step carcinogenesis has taken researchers a long way towards understanding the molecular events that underlie the transition from a normal cell to a transformed cell and neoplasia (1 ). This model has been extended to transgenic mouse studies where one of the carcinogenic events (an activated oncogene or loss of a tumor-supressor gene) is supplied in transgenic form. This permits an examination of the consequences of the transgene expression (or loss of expression in the case of null mice) during the evolution of malignancy.