KLRK1 and neoplasm: An imbalance in the activating and inhibitory cell surface receptors on NK cells such as NKG2D and CD161, low expression of the signal transducing ζ chain, and down-regulation of cytotoxic machinery caused by immunosuppressive cytokines such as IL-10 and TGF-β may explain the mechanisms behind NK cell dysfunction frequently observed in the tumor microenvironments [84].