ESR1 and breast cancer: In particular, the four subtypes that are mostly used to classify breast cancer are Luminal A (ER and/or PR+, HER2−, Ki67 low), Luminal B (ER and/or PR+, HER2−, Ki67 high), HER2 positive (HER2+) and triple negative (ER−, PR−, HER2−)2–5.