The role of HLA-DPB1 as transplantation antigen is controversial. The frequency and relevance of HLA-DPB1 mismatch in hematopoietic stem cell transplantation are unknown. To ascertain the rate of HLA-DBP1 mismatch in siblings that had been matched for HLA-A, -B, and -DRB1, reference strand mediated conformation analysis (RSCA) a high resolution HLA typing method was used. Locus-specific primers were used to amplify the HLA-DPB1 locus. The PCR product was then hybridized with two fluorescein-labeled references and the duplexes were analyzed after electrophoresis in a short polyacrylamide gel. Among the 113 pairs of individuals tested, six HLA-DPB1 mismatches were identified, which corresponds to a frequency of 5.31 % (95% confidence interval 3.20%-7.42 %).