Breast cancer is one ofthe most commonly diagnosed cancers andremains the leading cause of cancer-related deaths among women worldwide This type of cancer is classified into threesubtypes based on the expression levels of estrogen receptor (ER),progesterone receptor (PR), or human epidermal growth factor 2 receptor(HER2) in cancer cells: hormone receptor-positive (ER+ and/or PR+),HER2 positive (HER2+), and triple-negative breast cancer (ER-, PR-,and HER2-). The gene discussed is ESR1; the disease is cancer.