Breast cancer is the most common cancer type in women globally, with the following three subtypes: estrogen receptor or progesterone receptor-positive (ER+/PR+), which are treated with endocrine therapy; human epidermal growth factor receptor-positive (HER2+), which is treated with trastuzumab; and triple-negative tumors, which do not have any of the receptors and are associated with a high mortality rate. Breast cancers demonstrate high intratumoral heterogeneity, which makes a comprehensive molecular fingerprint even more important [29]. This evidence concerns the gene ESR1 and cancer.