While successful targeted therapies have been developed for the estrogen receptor and/or progesterone receptor expressing luminal A/B subtypes, and for the human epidermal growth factor receptor 2–amplified subtype, there is an unmet need for effective targeted treatment for breast cancers that lack the expression of these receptors [triple-negative breast cancer, (TNBC)], the clinical surrogate of basal-like breast cancer (BLBC) (3). This evidence concerns the gene ESR1 and breast cancer.