ESR1 and metastatic neoplasm: Although we found most metastatic tumors with PAM50 luminal classifications had high expression of ESR1 as expected, there was a subset of metastatic luminal tumors which harbored very low levels of ESR1 expression (Fig. 4d)—suggesting ESR1 depletion in endocrine-resistant luminal tumors is not restricted to local recurrences and may also be a feature of more advanced disease.