PDCD1 and neoplasm: There are many studies that are consistent with our speculation: Despite the fact that the mechanisms of resistance to anti-PD-1/PD-L1 therapy are complex, and largely unknown, it is reported that the immunosuppressive tumor microenvironment (TME) plays an important role in conveying resistance, and the mechanisms include hypoxia, increased inhibitory immune cells and immune molecules, and inhibition of immune-cell trafficking and infiltration (4, 7, 8).