Immunostaining showed that, while the mammary tumors of vehicle-treated WT-PyMT mice had high numbers of proliferative Ki67+ tumor cells, the numbers of dividing tumor cells were significantly lower in the vehicle-treated 18–/–-PyMT mice and in the lapatinib-treated WT-PyMT mice, and particularly low in the lapatinib-treated 18–/–-PyMT mice (Figure 8, F and H). Here, MKI67 is linked to breast cancer.