PDCD1 and esophageal squamous cell carcinoma: For advanced and metastatic ESCC, a new standard of treatment has evolved throughout the disease process based on a series of randomized clinical trials since the implementation of ICIs, including PD‐1/programmed death ligand 1 (PD‐L1) inhibitors and cytotoxic T‐lymphocyte‐associated protein 4 (CTLA‐4) inhibitors.