Engagement of PD-1 on T cells via its ligands PD-L1 or PD-L2 on the surface of lymphoid cells, macrophages, IFN-stimulated epithelial cells, DCs, or tumor cells blocks T cell receptor (TCR) signaling on cytolytic CD8+ cells via the recruitment of the SHP-2 phosphatase and dephosphorylation of the TCR complex (99–106). This evidence concerns the gene PDCD1 and neoplasm.