In the tumor microenvironment tumor cells and antigen presenting cells (APCs) upregulate PD-L1 expression through 2 pathways, adaptive immune tolerance and intrinsic immune tolerance, to evade immune surveillance, and PD-L1 upregulation is also a prerequisite for the efficacy of anti-PD-1/PD-L1 monoclonal antibodies. Here, PDCD1 is linked to neoplasm.