ESR1 and breast cancer: BC were classified based on the 2015 ESMO Guidelines [4] in five molecular subtypes: luminal A (estrogen receptor/ER+, HER2-negative, low ki67, high PR), luminal B human epidermal growth factor receptor 2 (HER2)-negative (ER+, HER2-, high ki67, or low PR+), luminal B HER2-positive (ER+, HER2+, any PR, any ki67), HER2-enriched (HER2+, ER-, PR-), and “basal like” or triple negative BC (ER-, PR-, HER2-negative/TNBC).