In human epidermal growth factor receptor 2 (HER2)-positive breast cancer treated with neoadjuvant systemic therapy with docetaxel, carboplatin, trastuzumab, and pertuzumab (TCHP), estrogen receptor (ER) expression, HER2 protein expression, and tumor-infiltrating lymphocyte (TIL) levels are significantly associated with pathologic complete response (pCR). Here, ESR1 is linked to breast cancer.