ESR1 and breast cancer: In this retrospective study of primary tumors from patients with ER+ breast cancer, including a main study cohort of 684 patients and a validation cohort of 273 patients, we identified high cancer cell levels of PD-L2 protein as an independent predictor of early recurrence after multivariable adjustment for age, race, histological grade, tumor size, node status, and radiation, chemotherapy, and hormone therapy.