An estimated 297 790 women will receive a diagnosis of breast cancer in the US in 2023, and nearly 43 170 women will die of the disease.1 Assessment of possible expression of estrogen receptor (ER), progesterone receptor (PR), and erb-B2 receptor tyrosine kinase 2 (ERBB2 [formerly HER2]) is standard in identifying the most suitable breast cancer treatment and determining prognosis. The gene discussed is ESR1; the disease is breast cancer.