PDCD1 and esophageal squamous cell carcinoma: Compared with other solid tumors, esophageal squamous cell carcinoma (ESCC) has a very high somatic mutation rate.61, 62 The high mutation load in esophageal tumors has been associated with the clinical benefit of PD‐1 blockade.63 Nivolumab is a human monoclonal immunoglobulin (Ig)G4 antibody that seals PD‐1 expressed on activated T cells.