ESR1 and breast cancer: In breast cancer cells, ERα is responsible for the proliferation and survival of cells due its two-fold action: via classical ERs activation and gene regulation through estrogen response elements (ERE), as well as via non-classical activation through signal transduction via Ras/mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K/Akt) [27].