PDCD1 and breast cancer: As the major ligand of PD-1, PD-L1 is widely expressed and correlated with poor prognosis in many human cancers, including melanoma [29], lung cancer [30], glioblastoma [31], breast cancer [7, 32], Wilms’ tumor [33], urothelial cancer [34], pancreatic cancers [35], esophagus adenocarcinoma [36], kidney tumors [37] as well as hematopoietic malignancies [38, 39].