PDCD1 and cancer: Immunotherapy can be an option for treating advanced biliary tract cancers based on the remarkable success of using immune checkpoint inhibitors (ICIs), including blocking antibodies against programmed cell death-1 (PD-1)/programmed cell death ligand-1 (PD-L1) and cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), in treating patients with several cancers [9,10,11].