DNA-based typing shows many HLA class I mismatches in the high CTLp frequency group not identified using serological methods. (▪) Matched pairs; (░) those with detected HLA class I mismatches. Forty-seven donors had high (>1:10⁵) patient-specific CTLp frequencies. Only 8 (17%) of these had HLA class I mismatches detected by serological typing, reflecting the low overall number of HLA class I serological mismatched pairs. The proportion of HLA class I mismatched pairs in the high CTLp frequency group increased to 23 (49%) after DNA-based typing, with 21 pairs mismatched at the HLA-B and/or -A locus. However, 24 pairs (51%) with high CTLp frequencies appear to be compatible for HLA class I.