PDCD1 and esophageal squamous cell carcinoma: Previously, three phase 3 clinical trials in which PD-1 inhibitors were tested as a second-line treatment in patients with ESCC who were unselected for PD-L1 expression revealed modest activity, as shown by objective response rates (ORRs) ranging from 16.7 to 20.2% (refs. 11–13).