A direct, physical correspondence between certain Pst I-generated genomic DNA fragments and Taq I-generated fragments, revealed with HLA-DQ alpha or -DQ beta gene probes, has been demonstrated. As an immediate consequence, the nature of the DQ and DX hybridized genes contained in the fragments was established. Taq I-generated DQ allelic forms which associate with serologically defined DR1, DR2, and DRw6 specificities were also proven to be sensu stricto "splits" of the Pst I-generated allelic form associated with all three DR specificities.