ERBB2 and breast cancer: Although HER2-low breast cancer patients had the lowest rate of lymph node metastasis, the subgroup analysis based on HR status showed no difference among different HER2 statuses in the HR+ or HR− subgroup, which suggested that the lower rate of lymphatic metastasis in HER2-low breast cancer was caused by the higher percentage of HR+ tumors (Table 1 and Additional file 3: Table S3).