Some human KIR haplotypes contain two KIR2DL5 genes: KIR2DL5A and KIR2DL5B.

Killer-cell immunoglobulin-like receptors (KIR) comprise a family of structurally diverse proteins encoded by a compact cluster of genes located in human Chromosome 19q13.4. The most recently described member of the KIR family, KIR2DL5, is represented in human populations by at least four gene variants, whose exons differ by two to eight nucleotides. We show here that these structurally similar variants are encoded by alleles of two different loci, KIR2DL5A and KIR2DL5B, which map to different regions of the KIR-gene cluster. Regarding KIR2DL5, four groups of KIR haplotypes can be distinguished: those having both KIR2DL5A and KIR2DL5B, those having either KIR2DL5A or KIR2DL5B, and those lacking KIR2DL5. Positive association between KIR2DL5A and KIR2DL5B was detected but did not reach statistical significance. These results are consistent with a model in which KIR2DL5A and KIR2DL5B are products of a gene duplication, which through the action of subsequent recombination have became separated on some haplotypes.