Translocations involving the T cell receptor alpha/delta (TCRalpha/delta) chain locus, which bring oncogenes in the proximity of the TCRalpha enhancer, are one of the hallmark features of human T cell malignancies from ataxia telangiectasia (AT) and non-AT patients. These lesions are frequently generated by the fusion of DNA breaks at the TCRalpha/delta locus to a disperse region centromeric of the immunoglobulin heavy chain (IgH) locus. Aberrant VDJ joining accounts for TCRalpha/delta associated DNA cleavage, but the molecular mechanism that leads to generation of the "oncogene partner" DNA break is unclear. Here we show that in ATM deficient primary mouse T cells, IgH/TCRalpha/delta fusions arise at a remarkably similar frequency as in human AT lymphocytes. Recombinase-activating gene (RAG) is responsible for both TCRalpha/delta as well as IgH associated breaks on chromosome 12 (Chr12), which are subject to varying degrees of chromosomal degradation. We suggest a new model for how oncogenic translocations can arise from two non-concerted physiological DSBs.