PDCD1 and biliary tract cancer: A subset analysis of the KEYNOTE-158 study revealed that treatment with pembrolizumab, a programmed cell death-1 inhibitor, for previously treated advanced biliary tract cancer with a deficiency in DNA mismatch repair/high microsatellite instability resulted in favorable outcomes, with a high response rate of 40% and a median survival time of 20 months [14,31].