PDCD1 and cancer: In cancers, including endometrial and ovarian cancer, the linkage of PD-1 to PD-L1 parallels with an environment rich in CD4+ CD25 high FoxP3+ T regulatory cells (T regs), high myeloid-derived suppressor cell (MDSC) activity, low cytotoxic T cell potential, and many other immunosuppressive parameters that trend to tumor progression [9,19,20,21].