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. CD158f is a 60 kD glycoprotein with 2 Ig-like extracellular domains and long cytoplasmic domains. Its expression is highly polymorphic between individuals and mainly expressed on a subset of NK cells and a small population of T cells. HLA class I alleles are the ligands of CD158f.