ESR1 and breast cancer: In the present studies utilizing MCF-7 (ERα+ HER2+) breast cancer cells which express EGFR/ERRB1 but not its ligand we have shown marked activation PRLR gene at the transcriptional level by exogenous EGF with the essential involvement of the mitogen-activated protein kinase (MAPK; ERK1/2) and PI3K-AKT signaling pathways mediated through activation of EGFR.