ERBB2 and breast cancer: Staging of BC into five stages on a number scale of 0 through IV is determined by the tumor-node-metastasis (TNM) system centered on the guidelines of the American Joint Committee on Cancer (AJCC).(4) The TNM system assigns the tumor stage based on the assessment of the anatomical extent of the disease, histological tumor grade, estrogen and progesterone receptor status, epidermal growth factor receptor (HER2), and the oncotype score.