To determine the chromosomal location of mouse immunoglobulin heavy chain structural genes unambiguously, a panel of somatic cell hybrids was scored for the presence of DNA sequences homologous to gamma 2b-, mu-, and alpha-heavy chain-constant region DNA probe molecules. The hybrids, formed between mouse and hamster cells, contained various combinations of mouse chromosomes plus a full set of hamster chromosomes. Hybrids that retained mouse chromosome 12 reacted with the probes, whereas hybrids that had lost the chromosome, or its distal half, failed to react. These results indicate that structural genes for the gamma 2b-, mu-, and alpha-heavy chain-constant regions map to the distal half of this chromosome.

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