All patients diagnosed in the Netherlands Cancer Institute between 2011 and 2019 with HER2-negative primary breast cancer with ER expression between 1 and 50% and for whom tumor material was available were identified, of which 17 patients had tumors with ER1-9% (low-positive) and 22 patients had tumors with ER10-50% (intermediate-positive; Supplementary Fig. 1). The gene discussed is ESR1; the disease is neoplasm.