PD-1 has also been identified directly on cancer cells in multiple tumor entities, such as melanoma5,7,12–19, non-small cell lung17,20,21, colorectal18,19,21, brain22,23, liver18,24–26, pancreatic17,27, gastric28, esophageal29–31, and thyroid cancers32,33. Here, PDCD1 is linked to neoplasm.