Recombination in the class III region of the mouse major histocompatibility complex

The sites of meiotic recombination in the class II region of the mouse major histocompatibility complex (MHC) are clustered at hotspots. To search for hotspots in the class III region, we mapped recombinational breakpoints of 79 Ab:H2-D recombinants with 11 DNA markers; these included Tnx, the gene for an extracellular matrix protein, tenascin X, the Notch-related Int3 gene, and a microsatellite marker, D17Mit13, none of which had previously been mapped precisely. The results gave the gene order of Eb-61.1-Int3-Tnx-Cyp21/C4-Bf-Hsp68c-D17Mit13+ ++-Tnfa/Tnfb-D. The crossover sites in 40 of the 79 recombinants were confined within the Eb/Int3:Tnx/Cyp21 interval. The result demonstrated that an unequal distribution of recombination is a general feature of the mouse MHC, suggesting the presence of a recombinational hotspot within the Int3:Tnx interval.