PDCD1 and esophageal squamous cell carcinoma: Interestingly, while PD-1 expression was confined to CD56dim NK cells in renal cell carcinoma patients [84] and Kaposi sarcoma patients [83], both CD56bright and CD56dim NK cells were found to up-regulate PD-1 in patients with hepatocellular carcinoma, esophageal squamous cell carcinoma, colorectal cancer, and other gastro-intestinal cancers [87].