The KIR family is composed of at least 15 members expressed by natural killer (NK) cells and a subset of T cells in primates only. KIR proteins are named based on the number of extracellular Ig-like domains (KIR2D or KIR3D) and for whether the cytoplasmic domain is long (L), or short (S). Inhibitory KIR proteins possess long cytoplasmic tails with ITIM (inhibitory) domains, and several of these molecules bind HLA Class I molecules or target cells. Activating KIR proteins have short tails; their targets are less well-known. KIR2DL4 (CD158d) is unique in that it shows both inhibitory and activating characteristics.