PDCD1 and nasopharyngeal carcinoma: Despite the abundance of tumour-infiltrating lymphocytes (TILs), many of these cells are highly activated and exhausted, expressing exhaustion markers such as lymphocyte activation gene (LAG-3), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), and programmed cell death 1(PD-1) (14), while high programmed death ligand 1 (PD-L1) expression on NPC cells promotes immune evasion (15).