Most recently, Judge et al. [115] compared PD-1 expression on mouse, human- and canine, T cells and NK cells under various conditions in vitro and investigated intratumoral NK cells in sarcoma, colon cancer, and head and neck squamous carcinoma, concluding that PD-1 was not significantly expressed on mouse or human NK cells. Here, PDCD1 is linked to colonic neoplasm.