PDCD1 and neoplasm: The engagement of the PD-1 receptor on the surface of T cells to PD-L1 ligands expressed on the surface of antigen presentation cells (APCs) or tumor cells will cause the conformational change of PD-1, leading to the phosphorylation of tyrosine in the ITIM and ITSM signal motifs in the intracellular domain of PD-1.