PD-1 (Programmed cell death protein 1) is an immune-suppressing receptor on immune cells, includes T cells, NK cells, etc. When it binds to PD-L1, which is highly expressed on the surface of tumor cells, it triggers the expression of SH2-containing protein tyrosine phosphatase 2 (SHP2), which in turn inhibits the PI3K/AKT axis. Here, PDCD1 is linked to neoplasm.