These immune cells infiltrate tumor tissues, reflecting the host’s immune reaction to the malignancy.1 In the context of human epidermal growth factor receptor 2 (HER2)-positive breast cancer, TILs are emerging as important biomarkers for both prognosis and treatment response.2 Studies have demonstrated that high TIL levels are associated with better survival outcomes in patients with advanced HER2-positive breast cancer treated with therapies such as trastuzumab and pertuzumab. The gene discussed is ERBB2; the disease is neoplasm.