ESR1 and breast cancer: As BC is a very heterogeneous disease with distinct morphologies, molecular traits, prognoses, and treatment options, clinical decisions are mainly made on the basis of the tumour stage, lymph-node involvement, and molecular subtype, represented clinically by the expression of estrogen receptor (ER), progesterone receptor (PR), HER2 receptor (HER), and the proliferation marker Ki-67 (Mib-1)3.