ESR1 and breast cancer: Consequently, four groups of BC have been described: Luminal A (ER+ or PR+, KI67-, HER2-), Luminal B (ER+ or PR+, KI67+ or HER2+), HER2 (ER-, HER2+) and triple negative (TNBC, ER-, PR-, HER2-) (for a review, see [5]).