PDCD1 and cancer: The binding of PD-1 to its ligand PD-L1 (expressed on various immune, endothelial, and cancer cells) or PD-L2 (expressed on APCs and can be induced in cancer cells) triggers the recruitment of domain-containing phosphatase 1 (SHP1) and SHP2, which leads to dephosphorylation of the TCR complex [17].