The HLA-B73 antigen has a most unusual structure that defines a second lineage of HLA-B alleles

The nucleotide sequence of cDNA encoding the HLA-B73 antigen was determined; it is unusually divergent, differing from other HLA-B alleles by 44–77 nucleotide substitutions. Features that distinguish the B*7301 heavy chain from other HLA-B heavy chains include multiple substitutions in the α3 domain and a duplication-deletion within the transmembrane region that increases the length of B*7301 compared to other HLA-B heavy chains. The duplication-deletion is shared with subsets of B alleles from the homologous gorilla (Gogo-B) and chimpanzee (Patr-B) loci. Other unusual features of B*7301 are individually shared with certain alleles of the HLA-A, HLA-C, HLA-F, Gogo-B and Patr-B loci. The B*7301 molecules has sequence elements in common with members of the B7 crossreacting group in the α1 domain and is shown to possess the ME1 epitope, which is held in common with the B7, B22, B27, B42 and B67 antigens. B*7301 has a unique cysteine at position 270 of the α3 domain which appears accessible but probably does not form disulphide-bonded B*7301 dimers in cell membranes. B*7301 represents a newly discovered but ancient lineage of HLA-B alleles that appears poorly represented in the modern human population.