At 24 months of age, none of the wild-type mice developed a tumor, whereas 10 cases of ductal hyperplasia and two tumors were observed in a total of 69 MMTV-Id1 mice in which constitutive Id1 overexpression retained well (17.39%, P = 0.001, chi-square test; Figure 3C). Here, ID1 is linked to neoplasm.