Map of genetic loci that can influence histocompatibility in the allogeneic HCT setting. The chromosomal location of the MHC, and of 2 other multigene clusters, the NKC and the KIR locus, are indicated by red labels and arrowheads to the left of the corresponding chromosomes. The chromosomal locations of genes that have been shown to encode T lymphocyte–defined minor H antigens are indicated by labels and arrowheads to the right of the corresponding chromosomes; genes that encode class I MHC-restricted minor H antigens recognized by CD8⁺ T cells are indicated by black labels, those that encode class II MHC-restricted minor H antigens recognized by CD4⁺ T cells are indicated by green labels, and those that encode both class I and class II MHC-restricted minor H antigens are indicated by blue labels.