Three Hsp70 genes are located in the C4-H-2D region: possible candidates for the Orch-1 locus.

The central region of the mouse MHC harbors a recombinational hot spot area. Most recombinations in this part of the complex take place between the Hsp70.1 gene and the G7 gene. This interval is of interest since structurally indistinguishable recombinant haplotypes do differ in functional behavior. Susceptibility to experimental allergic orchitis, which is controlled by the Orch-1 locus, is one example. We have analyzed the hot spot region at the molecular level in order to understand the molecular organization of this chromosomal segment. From a C57BL genomic library we constructed a cosmid contig bridging the interval between Hsp70.1 and G7. The Orch-1 gene maps to a 60-kb segment of DNA in which we found a new Hsp70 homologue, Hsp70.3. Thus, as in the human MHC, the central region of the mouse MHC harbors a cluster of three Hsp70 genes; Hsp70.1, Hsp70.3, and Hsc70t. Two other genes are located in this critical interval (G7b and G7a/Bat-6), and there might still be other undetected genes present in the region. Heat shock proteins play an important role in a large number of physiological processes and it is tempting to speculate that Hsc70t, which exhibits testis-specific expression, may be identical to Orch-1.