ESR1 and breast cancer: Significantly, many of the analyzed tumors were the ‘triple-negative (basal-like) breast cancers' (estrogen receptor (ER), progesterone receptor (PR) and erbB2 negative) that represent basal and mesenchymal mammary malignancies (Supplementary Figure 11).35 These tumors tend to be the most aggressive and represent a challenge to treat, and cytotoxic chemotherapy is currently the only systemic treatment option.