They generate multifocal adenocarcinomas in the mammary glands and develop lung metastases, but their onset of tumor formation and metastasis differ significantly; MMTV-PyMT mice start to develop primary tumors at 9 weeks of age and develop lung metastasis after 14 weeks of age, whereas MMTV-Neu mice start to develop primary tumors at 18 weeks of age and develop lung metastasis after 27–33 weeks of age [6, 7]. Here, ERBB2 is linked to neoplasm.