Programmed cell death 1 (PD-1) is a transmembrane protein expressed on T cells, B cells, and NK cells: PD-1 ligand (PD-L1) binds to PD-1 and then directly inhibits apoptosis of the tumor cell while promoting peripheral T effector cell exhaustion and conversion of T effector cells to Treg cells [54]. Here, PDCD1 is linked to neoplasm.