These include hormone receptors, such as estrogen receptor (ER) and progesterone receptor (PR), whose presence or absence informs the choice of hormonal therapy.41 HER2 is another pivotal biomarker, which has led to the development of HER2-targeted therapies like Herceptin, dramatically improving outcomes for HER2-positive breast cancer patients.42 This evidence concerns the gene ERBB2 and breast cancer.