PD-L2 has 2-6 fold higher affinity for PD-1 than PD-L1.25 PD-L2 has been associated with poor prognosis in some solid tumors, including hepatocellular carcinoma and clear cell renal carcinoma.26 However, although PD-L2 is frequently expressed by malignant breast epithelia,27 the prognostic value of PD-L2 expression in breast cancer remains largely unexplored. Here, PDCD1 is linked to breast cancer.