The estrogen bound ERα enters the nucleus and functions as a transcription factor binding to estrogen response elements (EREs) in the promoter regions of more than two-hundred genes, including the promoter of the PgR. Binding to EREs drives increased transcription of genes required for cell cycle progression in normal cells involved in female reproduction (26), as well as in breast cancer cells (27). Here, ESR1 is linked to breast carcinoma.