PDCD1 and esophageal squamous cell carcinoma: A sub-group analysis of the KEYNOTE 590 study implied that the OS of PD-1+DDP was prolonged in the esophageal adenocarcinoma sub-group, but this did not reach statistical significance, while in the esophageal squamous cell carcinoma sub-group, the OS of PD-1+DDP was significantly better than that of chemotherapy alone.