Cutting edge: expansion of the KIR locus by unequal crossing over.

The killer Ig-like receptor (KIR) genes have high sequence similarity and are organized in a head-to-tail fashion. These properties may enhance misalignment of homologous chromosomes during synapsis preceding meiotic recombination, resulting in unequal crossing over. We have identified an extended KIR haplotype that contains a novel hybrid gene and two copies of each of two previously described KIR genes. A parsimonious mechanism for the derivation of this haplotype invokes unequal crossing over between two known ancestral KIR haplotypes. These data raise the possibility that unequal crossing over may be responsible in part for the expansion/contraction of KIR haplotypes as well as other homologous gene families that map in tandem.