The Ϸ1-Mb leukocyte receptor complex at 19q13.4 is a key polymorphic immunoregion containing all of the natural killer-receptor KIR and related ILT genes. When the organization of the leukocyte receptor complex was compared from two haplotypes, the gene content in the KIR region varied dramatically, with framework loci flanking regions of widely variable gene content. The ILT genes were more stable in number except for ILT6, which was present only in one haplotype. Analysis of Alu repeats and comparison of KIR gene sequences, which are over 90% identical, are consistent with a recent origin. KIR genesis was followed by extensive duplication͞deletion as well as intergenic sequence exchange, reminiscent of MHC class I genes, which provide KIR ligands.

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The leukocyte receptor complex (LRC) on human chromosome 19q13.4 encompasses at least four families of related genes: immunoglobulin-like transcripts (ILT), killer cell inhibitory receptors (KIR), the leukocyte-associated inhibitory receptors (LAIR) and the Fc § receptor (Fc § R). We determined the genomic organization of a region of DNA spanning the junction of the ILT and KIR gene complexes. Extensive sequence data were collected for ILT3, two novel genes, ILT9 and ILT10, and one novel KIR locus (KIRCI). These loci, along with other reported sequences from the region, encoded a leader sequence split into two exons, upstream of two to four immunoglobulin (Ig) domains, each on a separate exon. Downstream of the Ig domains, however, the organization differs markedly between inhibitory and activating ILT. These data are consistent with a highly conserved gene arrangement for all superfamily members suggesting duplication of primordial sequences. ILT3 and KIRCI were in the same head-to-tail orientation as has been described for other KIR loci which may facilitate addition or loss of genes between different haplotypes.