ESR1 and breast cancer: Currently, genes that are epigenetically regulated via promoter methylation in breast cancer include cyclin-dependent kinase inhibitor (p16), breast cancer susceptibility gene 1 (BRCA1), estrogen receptor (ERα), progesterone receptor (PR), retinoic acid receptor-β2 (RARβ2), glutathione S-transferase p1 (GSTP1), E-cadherin, and tissue inhibitor of metalloproteinase 3 (TIMP3) [15].