ESR1 and breast cancer: However, due to the high cost of RNA-sequencing, immunohistochemistry (IHC) testing is the commonly used alternative method in clinics to identify the molecular characteristics of BC by detecting the gene expression of four specific biomarkers, including the estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and Ki67 [8,9].