The intervening years have also seen progress in the understanding of single molecular abnormalities in breast cancer related to specific molecular therapies, such as BReast CAncer (BRCA) gene, programmed cell death-1 ligand (PD-L1), phosphatidylinositol 3-kinase (PI3K), or estrogen receptor 1 (ESR1). This evidence concerns the gene ESR1 and breast cancer.