ESR1 and breast cancer: Following a stimulation with estrogen, nuclear-translocated ERα rapidly binds to estrogen response elements (EREs), which alter the expression of target genes, such as c-Myc, trefoil factor 1 (TFF1), Cathepsin D (CTSD), Growth Regulation by Estrogen in Breast cancer 1 (GREB1), and Progesterone Receptor (PgR)3–5.