Apparent HLA DR triplet due to the coexpression of a DRB5-encoded molecule on a DR1 haplotype

HLA DR1 molecules are coded by a single polymorphic DRB1 gene. We have observed rare DR1 cells in one Caucasoid family and three unrelated individuals that also reacted with some anti-DR2 sera. Since the second DR antigen was normally expressed, these cells appeared as triplets. Contrary to serology, the cells were not typed by HTCs defining Dw2, Dw12, and Dw21. Further investigations on these unusual DR1 + 2 ∗ haplotypes were conducted by DNA oligotyping and by sequencing of the DRB first-domain exon. The results showed that these DR1 haplotypes, besides their DRB1 ∗ 0101 allel, carried also a DRB5 ∗ 0101 allele.