PDCD1 and biliary tract cancer: At present, the clinical data on immunotherapy in CCA and other biliary tract cancers are limited and several trials are ongoing exploring, for instance, the role of monoclonal antibodies ipilimumab or tremelimumab (anti- CTL4) or antibodies targeting PD-L1, such as durvalumab, or its receptor programmed cell death protein 1 (PD-1), such as pembrolizumab or nivolumab [33,100].