PDCD1 and neoplasm: Although the blockade of PD-1/PD-L1 requires reactivation and clonal proliferation of the T-cells present in the tumor microenvironment, failure of the checkpoint inhibitors may lead to (1) insufficient generation of anti-tumor T cells, (2) inadequate function of tumor-specific T cells [66,67], or (3) impaired formation of T-cell memory [64,68].