Family studies define a new histocompatibility locus, SB, between HLA-DR and GLO

Gene products of the human HLA region are the principal histocompatibility barrier to human allogeneic tissue transplantation¹. In addition, HLA genes regulate immune responses to environmental antigens² and affect susceptibility to various diseases³. Although the HLA region has been studied intensively, only four histocompatibility loci (HLA-A, -B, -C and -DR) have been identified within this 'supergene', but the existence of additional loci has been suggested by immunochemical^4-9 and genetic recombination^10-15 studies. Recently, a new segregant series of five histocompatibility antigens, named 'Secondary B cell' (SB) antigens, was defined^16-18. We now report studies of 21 families which indicate that the gene encoding the SB antigens is closely linked to HLA-DR, but is distinguished from it by recombination in two families and can be mapped between HLA-DR and GLO in additional recombinant families.