PDCD1 and colorectal cancer: However, certain malignancies, such as gastro-esophageal and colorectal cancer, fail to respond to immune checkpoint inhibitor (ICI) therapy such as KeytrudaTM (anti-programmed cell death protein 1, anti-PD-1) or YervoyTM (anti-cytotoxic T-lymphocyte-associated protein 4, anti-CTLA-4) (3, 4).