To develop appropriate preclinical models, we used p53−/− mice as a sensitized background to generate Plk2−/−; p53−/− murine mammary tumors in a manner similar to that used previously to develop a bank of transplantable p53−/− tumors in Balb/c mice (ref. 54; Supplementary Fig. S4), the latter of which have been extensively characterized in our laboratories (55). The gene discussed is PLK2; the disease is breast cancer.