ESR1 and neoplasm: It can be assessed using various methods: by counting mitoses on tissue sections, by estimating the percentage of cells in the S-phase of the cell cycle (S-phase fraction; SPF) using flow cytometry, by detecting antigens (e.g., Ki-67) associated with proliferation by immunohistochemistry (IHC) and, in recent years, by checking for ER-positive tumours using genomic tests that include the expression of a number of proliferation genes (Oncotype Dx, Genomic Grade Index, and PAM50) [4,5,6,7].