KLRK1 and hepatocellular carcinoma: Accumulating data have shown that the ligands on HCC cells for NK-activating receptors, including natural cytotoxicity receptor 3 (NCR3; NKp30), natural killer receptor group 2 member D (NKG2D), and DNAM-1 proteins, are expressed at low levels, which is associated with the processes of disease [22,23].