Differences in HER2, estrogen receptor(ER), and progesterone receptor (PR) expression levels are used, inpart, to classify breast cancer cells as luminal A or B (ER+ and/orPR+, HER2– or HER2+), triple-negative/basal-like (HER2–,ER–, PR−), and HER2-enriched (HER2+, ER–, PR−),where this classification helps guide treatment.41 SK-BR3, MDA-MB-231, and MCF-7 breast cancer cell linesare examples of HER2 enriched, triple-negative, and luminal A types,respectively.42 Here, ESR1 is linked to breast cancer.