KRAS and neoplasm: Ki67 staining indicated that the proliferative potential of lung tumor cells in K-ras/COX-2−/− mice (Figure 3a) was lower than that in K-ras mice (Figure 3b) evidenced by the percentage of tumor cells with Ki-67–positive staining in K-ras/COX-2−/− mice (2.9%) was significantly lower than that inK-ras mice (6.2%; p < 0.05) (Figure 3c).