ESR1 and breast cancer: Of note, breast cancer can be classified into ER/PR+ luminal, human epidermal growth receptor 2-overexpressing (HER2+), and basal or triple negative subtypes24, and K19 is highly expressed in ER/PR+ or HER2+ subtypes that are luminal in origin in human breast cancer25, making MCF7 cell line a highly relevant cell line to study K19 function.