A, B, C and DRB1 are important for reducing the risk of GVHD and successful transplant outcomes; the A, B, C and DRB1 loci comprise 8 possible alleles (a haplotype being inherited from each parent); one additional locus, HLA-DQ, is also typed to ascertain haplotypes and assist in the search for a compatible donor; however mismatching at DQ has not been shown to be associated with adverse outcomes; high resolution molecular typing (at the allele level) is now the standard of care for unrelated donor searches and allows greater refinement of the search strategy