ESR1 and breast cancer: ER is the first and most prevalent breast cancer biomarker that is expressed at various levels in about 70%-75% of breast cancers, while the transmembrane receptor tyrosine kinase HER2 (Neu, ERBB2) is highly expressed, typically as a result of amplification of the genomic DNA containing HER2 and several other genes, in around 15%-20% of human breast cancer[3-5].