ESR1 and cancer: Parallel analysis of PD-L2 levels in ER-negative breast tumors showed higher PD-L2 levels in ER-negative than in ER+ breast tumors in both cancer (P < .001) and stromal cells (P < .001), but PD-L2 levels of a considerable fraction of ER+ tumors overlapped with ER-negative tumors (Data Supplement).