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TNM classification of malignant tumors (Breast Cancer Study Group). The eighth editions of the primary tumor, lymph node and metastasis classifications for breast cancer issued by the American Joint Commission of Cancer and the Union for International Cancer Control were revised in 2017. The major change made by the American Joint Commission of Cancer is to incorporate biological factors such as estrogen and progesterone receptor, human epidermal growth factor receptor 2, histological grade and multigene prognostic assays, into the staging system. Tumor biomarkers and low recurrence scores confirmed by multigene prognostic assays change the staging. Minor changes are to add the post-neoadjuvant therapy clinical and pathological classification and to define the size of tumor, lymph node and metastasis components more precisely. Little has changed in the Union for International Cancer Control. We have identified key points of change in both eighth editions of the tumor, lymph node and metastasis classifications and we discuss possible problems which may arise when they are adopted in Japanese practice, as well as future directions.