ESR1 and breast cancer: This cohort study used first-in-kind linkage of Surveillance, Epidemiology, and End Results, Medicare, and residual tumor repository data of 3522 women with newly diagnosed screening- vs symptomatic-detected estrogen receptor–positive nonmetastatic breast cancer to demonstrate that screening and socioeconomic factors remain associated with breast cancer outcomes, even after adjusting for clinical, demographic, and genomic factors.