Comparison of SNPs encoded on HLA-A1-B8-DR3, HLA-A3-B7-DR15, and HLA-A2-B7-DR15 haplotypes. SNP variation encoded on the 3 most common white haplotypes—HLA-A1-B8-DR3, A3-B7-DR15, and A2-B7-DR15—are a source of patient-donor variation. Data are shown for 28 HLA-A1-B8-DR3 chromosomes, 4 HLA-A3-B7-DR15 chromosomes, and 4 HLA-A2-B7-DR15 chromosomes. HLA-A1-B8-DR3 chromosomes share conserved sequences, indicated by the green bar; SNP substitutions are indicated by vertical black bars and black rectangles. HLA-matched and HLA-mismatched patients and donors may differ at SNP positions across the 7 Mb of the MHC. For example, a patient with the HLA-A1,3-B7,8-DR15,3 tissue type and donor with the HLA-A1,2-B7,8-DR15,3 tissue type (HLA-A mismatched) will have disparities at multiple nucleotide positions. These untyped SNPs may be a source of variation associated with increased risk of GVHD and TRM.⁴¹