PDCD1 and neoplasm: Studies have found that the drug resistance to the PD-1/PD-L1 monoclonal antibody may be associated with the following factors, the intrinsic immune evasion genetic pathways in tumor cells, tumor antigen mutations and presentation processes, interactions among multiple immune checkpoints, suppressive tumor microenvironment (tumor intrinsic factors, inflammatory environment), oncogenic pathways in tumors, epigenetic changes in key tumor proteins, accumulation of metabolites, etc [452].