Assessment of the nuclear proliferation biomarker Ki67 has multiple potential roles in breast and other cancers,1,2 either in standard clinical practice as a prognostic3–11 and predictive5,7,10,12 marker or in clinical trials as an eligibility criterion or as a primary end point in early-phase neoadjuvant studies.10 Perhaps the most critical use for standard clinical care would be to determine prognosis in the context of other factors, such as nodal status, tumor size, and estrogen receptor, progesterone receptor, and HER2 status. The gene discussed is ESR1; the disease is neoplasm.