PDCD1 and biliary tract cancer: In phase 2 studies, anti-programmed cell death protein 1 (anti-PD-1) antibody nivolumab demonstrated complete or partial remission (objective response) in 22% of biliary tract cancer patients [8], and nivolumab combined with anti-cytotoxic T lymphocyte antigen 4 antibody ipilimumab induced an objective response in 31% of patients with intrahepatic CCA [9].