ESR1 and breast cancer: Some researchers might tend to use clinical phenotypes as surrogates for breast cancer molecular subtypes such that ER-positive tumours were analogous to luminal subtype, tumours with ER-negativity/HER2 over-expression were analogous to HER2-enriched subtype, and tumours with ER-negativity/HER2 negativity were analogous to basal-like breast cancer [15,16].