The present invention relates to a novel protein, termed NTB-A, nucleic acid molecules encoding the same and uses thereof. The invention also relates to methods of regulating Natural Killer cells activity by regulating the activity of NTBA-A in vitro, ex vivo or in vivo. The invention also comprises methods of screening active compounds using NTB-A or fragments thereof, or nucleic acid encoding the same, or recombinant host cells expressing said polypeptide.