The 15;12 translocations in murine plasmacytomas represent recombination of the myc oncogene with the immunoglobulin CH locus, often within CH switch recombination (SH) regions. Chromosome junctions cloned from four plasmacytomas confirmed that the translocation generates reciprocal CHSH-myc and 5′myc-SH structures. H locus targets included Sα fused to Sμ, Sμ fused to Sγ2b, and a germline Sα region. The nature of two H locus targets suggests that the target need not be highly active transcriptionally. Switch recombination machinery is implicated in the translocation by the SH targets and by homology of certain c-myc breakpoints with normal switch recombination sites. Fusion regions revealed deletions, extraneous nucleotides, and one duplication. These results prompt a translocation model in which staggered single-stranded breaks on each chromosome are followed by single-strand excision or polymerization prior to ligation to the other chromosome.