AbstractA new congenic mouse strain (C57BL16‐Igh‐Vb‐Ca) with a recombinant chromosome 12 is described. It carries the Igh‐laallele, but shows the serological characteristics of C57BL/6 when analyzed for idiotype expression with respect to the antigens dextran and (4‐hydroxy‐5‐iodo‐3‐nitrophenyl)acetyl (NIP). We analyzed liver DNA from one animal for restriction fragment length polymorphism by hybridization to probes detecting members of nine VHgene families and DHsegments, and compared it to DNA from animals carrying the nonrecombinant haplotypes Igh2and Ighb, respectively. The breakpoint of recombination maps to the region carrying members of VHgene families VGAM3.8, PC7183 and Q52. The CB8KN strain which according to the serological analysis carries a recombinant Igh locus (Igh‐Va‐Cb) on BALB/c background was also analyzed. In this strain the breakpoint of recombination again maps to the region carrying members of VHgene families VGAM3.8, PC7183 and Q52. Our results show that the VHgenes of families PC7183 and Q52 are interspersed and map to the region next to the DHlocus. At least one gene from the VGAM3.8 family also maps to this region in the Igha
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