ESR1 and breast cancer: Phase I and II trials with relatively small numbers of patients have also supported the importance of other biomarkers that were a priori hypothesized to be important (e.g., estrogen receptors for tamoxifen in breast cancer [77], Her-2/neu overexpression for trastuzumab in breast cancer [78], BCR/ABL fusion genes for imatinib in chronic myelogenous leukemia [79], c-KIT mutations for imatinib in gastrointestinal stromal tumors [80], BRAF v600E mutations for selected BRAF inhibitors in malignant melanoma [81], and PD-L1 expression for an anti-PD-1 antibody [82]).