Although genotyping is becoming increasingly sensitive to allelic variation, GVHD can still develop in 10/10 matched unrelated donors suggesting that a deeper understanding of the mismatches in HLA class Ib alleles (HLA-E, -F, -G and -H), HLA-DM (despite their relatively low expression and allelic diversity) and millions of minor histocompatibility antigens (e.g. H-Y) may be necessary to fully understand a patient’s susceptibility/probability of developing GVHD (11). Here, HLA-E is linked to graft versus host disease.