MKI67 and neoplasm: The data revealed that MUT-PyMT accelerated development of highly proliferative mammary carcinoma at 5 weeks of age when WT-PyMT had only pre-malignant lesions (Fig. 4a, Supplementary Fig. 3a–c, tumor-free survival: WT-PyMT 8 weeks vs. MUT-PyMT 5 weeks; Ki67-positive cells: WT-PyMT 3.7% vs. MUT-PyMT 11.7%).