PDCD1 and esophageal squamous cell carcinoma: Recently, there have been breakthroughs in the treatment of ESCC patients with Immune Checkpoint Inhibitors (ICIs) targeting programmed cell death protein 1 (PD1), programmed cell death 1 ligand 1 (PDL1), or cytotoxic T lymphocyte antigen 4 (anti-CTLA4) [5, 6].