ESR1 and breast neoplasm: Mutations affecting the ESR1 gene have been detected mainly in metastases from ER-positive/HER2-negative breast tumors, at a frequency ranging from 5 to 25%, when considering series with >20 interrogated metastases.5,13–18 The largest cohort of BC metastases with information about the ESR1 mutational status is the one derived from the prospective clinical sequencing initiative from the Memorial Sloan Kettering Cancer Center (MSKCC-IMPACT18), which reported mutations in 107/835 (12.5%) metastatic BC patients (source: cbioportal.org19,20).