However, attempts to correlate responses with PD-L1 have generally failed, although pCR rates were higher in PD-L1–positive patients in the KEYNOTE-522 trial19,20,24,34 while PD-L2 has largely been overlooked.23 Considering the reported 2-6 fold greater affinity of PD-L2 for PD-1 than PD-L125 and our new data documenting frequent high expression of PD-L2 on ER+ breast cancer cells, PD-L2 expression may be at least as important as PD-L1 in ER+ breast cancer. This evidence concerns the gene PDCD1 and breast cancer.