KLRK1 and cancer: Among them, the interaction between NK receptor group 2, membrane D (NKG2D) activating receptor and its ligand, such as MHC I-related chain (MIC) A/B and UL16 binding protein (ULBP) 1–5, seem to play a vital role in NK cell cytotoxicity [4] against cancer cells as ‘induced self’, although the tumors can secret these ligands as a soluble form to evade the immune surveillance.