KIR A and KIR B haplotypes of the human KIR locus. Human KIR haplotypes differ in their content of KIR genes and in the relative number of genes coding for activating and inhibitory KIR. Although the human population has numerous different KIR haplotypes they divide into 2 functionally distinctive groups. These group A and B KIR haplotypes exhibit different correlations with a spectrum of diseases, as well as the outcomes of HCT and other forms of immunotherapy. Shown are gene maps for 2 A and 2 B KIR haplotypes, which represent the overall diversity of KIR haplotypes. Each box represents a KIR gene, for which the shading gives the nature of the encoded protein: green, activating KIR; orange, inhibitory KIR; black, KIR of unknown function: gray, pseudogene, no KIR. Human KIR are of 4 evolutionary lineages, which are distinguished by the color of the label in the gene box: white, lineage I; yellow, lineage II; dark blue, lineage III; cyan, lineage V. The zigzag joining the centromeric and telomeric regions is an extended repetitive sequence and a hotspot for reciprocal recombination. Within the telomeric and centromeric regions the KIR genes are separated by short homologous sequences of a few hundred base pairs.