ESR1 and breast cancer: Breast cancer patients are divided into the following four therapeutically-relevant subtypes on the basis of HER2, estrogen receptor (ER) and progesterone receptor (PR) expression levels in tumor cells; luminal A (HER2-, ER+ and/or PR+), luminal B (HER2+, ER+ and/or PR+), HER2-enriched (HER2+, ER-, PR-) and TNBC (HER2-, ER-, PR-).