Figure 6. Comparison of the organization of the gene families encoding human KIR and mouse Ly49 NK cell receptors.

Human KIR and mouse Ly49 receptors serve similar functions as highly diverse NK cell receptors for MHC class I. However these functions have evolved by convergent evolution, because Ly49 are lectin-like receptors encoded in the natural killer cell complex (NKC) and KIR are immunoglobulin-like receptors encoded in the leukocyte receptor complex (LRC). Despite their independence the gene families encoding mouse Ly49 and human KIR have a similar organization with a framework defining two regions of gene-content variability. Shown in the figure are the most common gene-content motifs in the centromeric and telomeric regions of human KIR haplotypes in Caucasian populations [112, 19], and the organization of the four mouse Ly49 haplotypes sequenced [69]. In the human genome the Ly49 gene family is represented by one pseudogene [113, 114]; in the mouse genome the KIR family is represented by one pseudogene and one functional gene that is not expressed by NK cells [63]. A variety of human haplotypes with different gene-content from those shown in the figure have been described and are the results of gene duplication or deletion arising from non-homologous recombination. Such haplotypes are either rarer than those comprising the common gene-content motifs, and/or restricted to non-Caucasian populations [115-120].