The HER2-low group showed lower histologic grade (p < 0.001), fewer tumor-infiltrating lymphocytes (p < 0.001), higher ER positivity rate (p < 0.001), higher PR positivity rate (p < 0.001), and lower Ki-67 proliferation index (p < 0.001) than the HER2-positive group. The gene discussed is ESR1; the disease is neoplasm.