Three families of human Ia molecules, DP, DQ, and DR, have previously been defined. A cDNA clone, pDSH-9.1, encoding the a chain of a DQ molecule derived from an HLA-DRw9 homozygous cell line has been isolated and sequenced. Comparison of the nucleotide and predicted protein sequence to those of other DQ a subunits reveals that DQ a subunits derived from DR4, -7, and -9 cells are very similar to each other but quite different from a DQ a subunit derived from a DRw6 cell line. These studies suggest that certain Ia haplotypes have a common evolutionary history. Furthermore, in the context of current serologic and biochemical knowledge, they suggest that the gene encoding the DQ a subunit is in strong linkage disequilibrium with the DR locus.The Ia region of the major histocompatibility complex (MHC) consists of a series of closely linked loci encoding cell surface molecules that regulate the immune response. These Ia molecules may be divided into three groups, DP (formerly known as SB) (1), DQ (formerly known as DS) (2), and DR (3), that are structurally distinct. Ia molecules consist of two subunits, a and ,3, with molecular weights of approximately 28,000 and 33,000. Within each group, DP, DQ, and DR, multiple a-and /8-chain gene loci have been described; at present seven P3 chain (two DP, two DQ, and three DR) and five a chain (two DP, two DQ, and one DR) gene loci have been identified (4-9). Although their precise genetic order is not yet known, the DP locus is known to be centromeric to both DQ and DR, which in turn are centromeric to the class I loci, HLA-A, -B, and -C (10). Recombinational analysis indicates that both DP and the class I loci are approximately 1 centimorgan from the DQ and DR loci (10,11). No recombinational event between DQ and DR has yet been confirmed, suggesting a relatively low frequency of recombination. Biochemical studies (2,(12)(13)(14)(15)(16)(17)(18) and analysis of cDNA clones (19)(20)(21)(22)(23)(24)(25)(26) indicate that a majority of the Ia genes are expressed as products. Ia gene products belonging to the same group are highly related (greater than 95% protein homology) while those belonging to different groups are only approximately 60% related.The Ia system is highly polymorphic. At least 11 alleles at the DR locus (designated numerically-i.e., DR1-DR11) have been defined serologically (27). Similarly, at least 6 alleles at the DQ locus and 6 at the DP locus have been defined biochemically and by cellular assays, respectively (27,28 (29). Similarly, cells typing as DR1, -2, and -6 frequently display the MB1 antigenic determinant (30).Although the molecular basis for these antigenic determinants is not known, in some instances, the gene products expressing them have been identified. Thus, MB1, which is expressed on Drl, -2, and -6 cells, and MB3, which is expressed on DR4 and DR5 cells, are both present on different allelic forms of DQ molecules (28,31,32). In other instances, the molecular assignment has been more controversial; MT3 has been localized to DR-like mol...