Similar observations were obtained using aHY TCR Tg RAG-2-and OT-1 TCR Tg RAG-1 deficient strains. If in the case of the RAG-2-deficient mice it is conceivable that RAG-1 alone could perform VDJ recombination, this hypothesis is very unlikely for RAG-1-deficient mice. However, two RAG-1 knockout alleles have been generated and the RAG-1 KO strain we have analyzed here has the potential to be a hypomorphic allele due to the remaining expression of the essential catalytic RAG-1 core. In order to confirm the expression of endogenous TCRs by the donor RAG-2-deficient T-cells in the allogeneic hosts, we analyzed their TCRa and b chain usage by Immunoscope. We found that CD4 and CD8 T-cell populations expressed multiple non-Tg a and b TCR-chain transcripts with significant Complementaring Determining Region 3 length complexity. Interestingly, we were unable to detect Va11- and Vb3-peaks other than those corresponding to the Tg-encoded transcripts. This observation suggests the presence of dual TCR expressing Tcells where the abundant Tg chain message may have masked the detection of the endogenous rearranged Va11- and Vb3transcripts. The finding of Vb3-Cb transcripts expressing Jb gene Eupalinilide-C segments different from the Jb Tg confirmed this hypothesis. It is important to note that we confirmed the RAG-deficiency in all donor and host mice both by Flow Cytometry and by a 35�C40 cycle PCR. We should add that by using the same PCR and Immunoscope conditions, we were unable to detect any TCRb mRNA transcripts among the spleen cells of TCRb gene enhancer0/0 mice. Taken together, these findings indicate that transfer of TCR-Tg T cells from 5CC7 RAG-2-deficient donors into allogeneic hosts, resulted in the ����selection���� of donor T cells expressing diverse endogenous TCRs. Endogenous TCR expression in the 5CC7 RAG-2-deficient mice could be mediated either by RAG-dependent recombination, gene conversion or gene replacement mechanisms involved in the generation of BCR and TCR diversity. To discriminate between these possibilities, we sequenced some of the TCRa and Ophiopojaponin-C TCRb-chains used by T-cells recovered from the allogeneic hosts.