ESR1 and breast cancer: Several biomarkers, including HER2 amplicon, phosphoinositide-3 kinase signaling, estrogen receptor (ER) status, phosphatase and tensin homolog (PTEN) loss, insulin-like growth factor 1 receptor (IGF1R) signaling, and immune cell signatures, which are identified in primary breast tumors, have been associated with the trastuzumab sensitivity in early breast cancer patients undergoing neoadjuvant or adjuvant treatment5–7.