Briefly, PD-1 is a negative co-stimulatory transmembrane protein expressed on T-cells, B-cells, and NK cells; its binding to PD-L1 (which is expressed on tumor cells) and PD-L2 leads to peripheral T effector cell modulation as well as to tumor cell apoptosis and increased conversion of T effector cells to Treg cells [118]. Here, PDCD1 is linked to neoplasm.