We analyzed newly diagnosed invasive breast carcinomas from the pathology database at Dartmouth-Hitchcock Medical Center (DHMC) from 2012–2020 (n = 2208) and observed 46 (2.1%) that were classified as low-ER tumors containing between 1–10% ER-expressing tumor cells (Fig. 1A). Here, ESR1 is linked to invasive breast carcinoma.