PDCD1 and neoplasm: PD-1 by its ligand PD-L1, expressed on antigen-presenting cells and tumor cells (6), induces negative signaling that counters signaling through the antigen-specific T cell receptor (TCR) and the costimulatory protein CD28 (7), leading to inhibition of T cell activation, proliferation, and cytokine production, and ultimately to apoptosis (8).