PDCD1 and neoplasm: Interaction between programmed cell death protein 1 (PD-1) and PD-2 on the surface of T cells and its ligand PD-L1 on the surface of tumor cells or APCs can drive T cell fates toward Tregs through ZAP70 inhibition by SHP2 and attenuates all three classic T cell activating pathways, namely MAPK, inositol 1,4,5-trisphosphate (Insp3), and diacylglycerol (DAG).159