CD158 molecules, also known as KIRs (killer cell immunoglobulin-like receptors), are a family of transmembrane proteins with either two (KIR2D) or three (KIR3D) Ig-like extracellular domains. Some KIRs with long cytoplasmic domains contain ITIMs and posses inhibitory functions and others with short cytoplasmic region lack ITIM and have activation functions. 14 polymorphic KIR genes have been reported in humans. CD158 is mainly expressed on a subset of NK cells and a small population of CD8+ T cells. The mAb HP-MA4 reacts with KIR2DL1 (CD158a) and KIR2DS1 (CD158h). HLA-C is the ligand of CD158a/h.