Recombination in mouse cells was analyzed using extrachromosomal DNA substrates carrying the mouse immunoglobulin switch regions Sμ and Sγ2b. Recombination was detected at a frequency of 10 −2–10 −3 in mouse fibroblasts and in pre-B cell lines, but at a low frequency in a scid fibroblast cell line. Restriction enzyme digestion profile revealed that most recombination occurred between the CMV promoter region, which neighbors the Sμ upstream region, and the Sγ2b region. However, frequency of direct recombination between the CMV promoter region and the Sy2b region was low as measured by the substrate-lacking SR region. Nucleotide sequence analysis showed that recombination occurred between several homologous base-pairs, and extranucleotides were frequently found at the recombination junctions. These results indicate that recombination took the form of the recombination mediated by double-strand breaks. Double-strand breaks likely occurred in the Sμ and/or Sγ2b region, and the ends joined.