ESR1 and metastatic neoplasm: Confirmatory biopsy of breast cancer metastases may have many potential benefits: confirming metastatic disease; disclosing non-malignant disease or other primary tumors; and confirming concordance (or discordance) of biological features of disease such as estrogen receptor (ER), progesterone receptor (PgR) and human epidermal growth factor receptor (HER)2 status.