NKp30/NCR3, NKp44/NCR2, and NKp46/NCR1 are termed natural cytotoxicity receptors (NCR). These receptors are type I transmembrane proteins with 1-2 extracellular immunoglobulin domains, a transmembrane domain containing a positively charged amino acid residue, and a short cytoplasmic tail. All are expressed almost exclusively by NK cells and play a major role in triggering NK-mediated killing of most tumor cell lines. NKp44 is induced upon activation of resting NK cells with IL-2 and can bind to viral hemagglutinins. NK-mediated dendritic cell (DC) recognition is highly dependent on engagement of NKp30 and may result in lysis of the DC.