Killer cell immunoglobulin-like receptors (KIRs) are polymorphic transmembrane glycoproteins expressed by natural killer cells and subsets of T cells. They are classified by the number of extracellular immunoglobulin domains (2D or 3D) and by whether they have a long (L) or short (S) cytoplasmic domain. KIR proteins with the long cytoplasmic domain (such as CD158a / KIR2DL1) transduce inhibitory signals upon ligand binding via an immune tyrosine-based inhibitory motif (ITIM), while KIR proteins with the short cytoplasmic domain (such as CD158g / KIR2DS5, CD158h / KIR2DS1, or KIR2DS3) lack the ITIM motif and instead associate with the TYRO protein tyrosine kinase binding protein to transduce activating signals. The ligands for CD158 isoforms are subsets of MHC class I molecules.