ESR1 and cancer: Trastuzumab and pertuzumab without any chemotherapy can result in 6% pCR rate in ER-positive/ERBB2-positive cancers,5 whereas the combination of trastuzumab and pertuzumab with sequential anthracycline and taxane chemotherapy can result in pCR rates as high as 80% in ER-negative/ERBB2-positive cancers.6,7 Equally importantly, the decreased survival rates of patients with residual ERBB2-positive disease compared with individuals with pCR can be improved by additional, adjuvant therapy with trastuzumab emtansine (T-DM1).