and *4501, but not other alleles. Based on these findings, we developed an algorithm for prediction of nonpermissive HLA-DPB1 mismatches. Retrospective evaluation of 118 transplantations showed that the presence of nonpermissive HLA-DPB1 mismatches was correlated with significantly increased hazards of acute grade II to IV graftversus-host disease (HR ؍ 1.87, P ؍ .046) and transplantation-related mortality (HR ؍ 2.69, P ؍ .027) but not relapse (HR ؍ 0.98, P ؍ .939), as compared with the permissive group. There was also a marked but statistically not significant increase in the hazards of overall mortality (HR ؍ 1.64, P ؍ .1). These data suggest that biologic characterization of in vivo alloreactivity can be a tool for definition of clinically relevant nonpermissive HLA mismatches for unrelated HSC transplantation.