ESR1 and cancer: Previous studies reported differential expression of most important biomarkers of breast cancer (ER, PR, and HER-2) between primary cancer and metastatic cancer, with 21–50% showing HER-2 loss, about 30% showing HER-2 gain [8,32], 3.2–44% showing ER loss [33-35], 24% showing PR loss [35], and ER or PR gain not reported.