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The human major histocompatibility complex contains the genes for at least three different types of class II antigens, DR, DC and SB (DR, DQ and DP). They are all composed of an alpha and a beta chain. We have cloned a chromosomal region of 70 kb containing the SB (DP) gene family in overlapping cosmid clones. This segment contains two alpha genes and two beta genes, located in the order SB alpha 1, SB beta 1, SB alpha 2 and SB beta 2. The orientation of the alpha genes is reversed compared with that of the beta genes. This organisation suggests that the SB region has arisen by duplication of a chromosomal segment encompassing one alpha and one beta gene. Partial nucleotide sequences of the SB alpha 1 and SB beta 1 exons demonstrate that the genes correspond to SB alpha and beta cDNA clones. Consequently these genes are expressed. In contrast nucleotide sequence determination of the …