EXPERIMENTAL FIGURE 24-11 The kinetics of tumor appearance in female mice carrying either one or two oncogenic transgenes shows the cooperative nature of multiple mutations in cancer induction. Each of the transgenes was driven by the mouse mammary tumor virus (MMTV) breast-specific promoter. The hormonal stimulation associated with pregnancy activates the MMTV promoter and hence the overexpression of the transgenes in mammary tissue. The graph shows the time course of tumorigenesis in mice carrying either MYC or ras^V12 transgenes as well as in the progeny of a cross of MYC carriers with ras^V12 carriers, which contain both transgenes. The results clearly demonstrate the cooperative effects of multiple mutations in cancer induction. See E. Sinn et al., 1987, Cell 49:465.