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 alpha 3 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 alpha 1 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 alpha 3 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.