ESR1 and breast cancer: Numerous ERα downstream pathways have been identified, of which the proto-oncogene tyrosine-protein kinase SRC, cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA), mitogen-activated protein kinase (MAPK), phosphatidylinositol 3 kinase (PI3K)/AKT, and phospholipase C (PLC)/protein kinase C α (PKCα) are the most crucial signaling pathways that sustain breast cancer development and progression [6].