ESR1 and breast cancer: However, in clinical practice, the combination of expression of hormone receptors and HER2 by immunohistochemistry (IHC) is more commonly used to define breast cancers into the luminal A (ER+ or PgR+, HER2-), luminal B (ER+ or PgR+, HER2+), HER2-overexpressing (ER- and PgR-, HER2+), and triple-negative (TNBC: ER-, PgR-, HER2-) subtypes, which demonstrate major differences in clinical outcomes, with the luminal A subtype showing the best prognosis [13,14].