Receptor on natural killer (NK) cells for MHC class I molecules. Upon interaction with peptide-free HLA-F open conformer, triggers NK cell degranulation and anti-viral cytokine production. Interacts with HLA-F open conformer; this interaction is direct. Expressed in NK and T-cell lines but not in B-lymphoblastoid cell lines or in a colon carcinoma cell line. The KIR genes are located in a segment of DNA on 19q13.4 in the leukocyte receptor complex that has undergone expansion and contraction over time, probably through unequal crossing-over. Thus, KIR haplotypes vary in the number and types of genes, although a few framework loci, such as the gene KIR3DL1, are present on all or nearly all haplotypes. KIR3DL1 and KIR3DS1 segregate as alleles of the locus KIR3DL1/3DS1. Belongs to the immunoglobulin superfamily.