In our study, the expression of Rrm2 increased throughout the breast cancer development in PyMT mice (log2 fold change in hyperplasia, adenoma/MIN, early carcinoma, late carcinoma is 2.5, 4.0, 2.7, and 2.8, respectively) and may serve as potential biomarker for diagnosis as well as drug target. The gene discussed is RRM2; the disease is hyperplasia.