ESR1 and breast cancer: Approximately 60-70 % of breast cancers express the estrogen-dependent ERα transcription factor which when bound by estrogens, such as 17-ß estradiol (E2), regulate cell cycle target genes including CDK2, CDK4, Cyclin D1 and the proto-oncogene c-Myc to promote cancer cell proliferation (Ciocca and Fanelli, 1997[17]; Dalvai and Bystricky, 2010[18]).