Key molecular biomarkers such as estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) drive treatment decisions in primary breast cancer [48], and mutations in genes like ESR1, PIK3CA, BRCA1, and BRCA2 inform personalized therapies [49,50,51,52]. The gene discussed is ESR1; the disease is breast cancer.