CD226 and neoplasm: NK cells can eliminate malignant tumors in a non-MHC and non-tumor antigen-restricted manner through an array of activating (i.e., NKG2D, DNAM-1, NCRs) and inhibitory receptors (i.e., KIRs, NKG2A-CD94) that detect changes in the expression of their ligands during viral infection and malignant transformation.