Natural killer (NK) cells mediate MHC nonrestricted cytotoxicity. There are many candidate molecules for NK cell recognition structures, including inhibiting and activating receptors. Using a monoclonal antibody capable of triggering NK-mediated lysis, Sivori et al. (1997) identified a cell surface molecule of 46 kD that they termed p46. The molecule lacks N-linked sugars and is expressed on resting and activated NK cells but not on T cells or B cells. In addition to lytic activity, monoclonal antibody-mediated crosslinking induces intracellular calcium increases and cytokine production.