ESR1 and breast cancer: The use of molecular markers, such as the receptor status of estrogen (ERα), progesterone (PR), epidermal growth factor 2 (Her2/neu), and proliferation marker Ki67, can be helpful to subclassify BC cases into: Luminal A (ERα-positive, PR-positive, Her2/neu-negative, and Ki67-low), Luminal B (ERα-positive, PR-positive, Her2/neu-positive or negative, and Ki67-high), Her2/neu-positive (ERα-negative, PR-negative, and Her2/neu-positive), and triple negative (TNBC) (negative for ERα, PR, and Her2/neu).