KLRK1 and viral infectious disease: Among those receptors, NKG2D is critical for cancer immunosurveillance [23], since immune cells expressing this receptor recognize and eliminate target cells expressing specific ligands (MICA/B and ULBP1/2/3/4/5/6 ligands) that are upregulated in cells exposed to stress stimuli, such as after viral infection, heat shock, and malignant transformation [24,25].