ESR1 and breast cancer: As such, BC is grouped into Luminal A (ER-positive (ER+), PR positive, HER2 negative, Ki67 <14%) (6), Luminal B‐like (HER2‐negative) tumors (ER+, HER2 negative, and Ki67 ≥14%), Luminal B‐like (HER2‐positive [HER2+]) tumors (ER+, HER2+, any Ki67 level), HER2+ (non‐luminal) tumors (HER2+ and ER and PR negative) and finally triple‐negative (ductal) tumors are defined as ER, PR, and HER2 negative (7, 8).