ESR1 and breast cancer: This patient did not have a history of ER + breast cancer and this mutation likely reflects a sub-clone of ER + cells that is a rare event as in previous studies the ESR1 recurrent mutations were not detected in TNBC metastatic tissue samples.2 In the subgroup of patients with ER + /HER2-metastatic breast cancer, 30% were found to have an ESR1 mutation and 26% of these patients had more than one ESR1 mutation suggesting polyclonal disease (Fig. 1c).