We followed tumor growth in a cohort of littermate MMTV-PyMT;Mmp13+/+ (n = 38) and MMTV-PyMT;Mmp13−/− females (n = 25) by weekly palpations and caliper measurements until 13 weeks of age, when tumors were isolated for histological examination and the lungs isolated to determine metastasis burden. Here, MMP13 is linked to neoplasm.