The sequences of the endogenous Va10 and Vb8.1 TCR chains evidenced the usage of diverse Ja,D b and Jb segments. The number of genes involved argues against a gene replacement mechanism, that could only occur if different Tg copies were inserted in proximity to both the TCR-a and TCR-b loci, an unlikely event. The perfect alignment between the V sequences and the corresponding germ line sequences are inconsistent with the insertion of V sequence fragments, a hallmark of gene conversion. The sequences also showed considerable CDR3 length diversity using different D and 8-Gingerol J-gene segments and containing motifs reminiscent of N/P additions. Overall our findings suggest that RAG-dependent endogenous TCR recombination occurs in T-cells from 5CC7 donor mice despite their RAG-2-deficiency. This indicates that endogenous TCR recombination in T cells from TCR Tg RAG-deficient mice is not restricted to a particular TCR Tg strain. The nature of the TCR sequences obtained strongly support RAGlike activity. Therefore our results suggest that in some of the currently available TCR Tg RAG-deficient mice a single RAG protein could induce VDJ recombination. Although it is acceptable that RAG-1 alone could perform VDJ recombination since it has both specific DNA binding domains and catalytic sites for DNA cleavage, this hypothesis is very unlikely for RAG-2 since it lacks both DNA binding Timosaponin-BII activity and catalytic activity. It is possible that the RAG gene manipulations used to produce the currently available RAG-1-deficient mice could have lead to hypomorphic forms resulting in residual recombination activity. This may occurs in the RAG-1 deficient mice used here, due to a Neo-gene insertion at the position aa330 that could allow residual recombinase activity of the RAG-1 protein. Thus, the present report challenges the traditional view that both RAG-1 and RAG-2 proteins are strictly required to ensure V J recombination, and indicate that RAG-1 alone may be sufficient to induce, although at very low level, some TCR recombination.In these conditions, one can not exclude that the widely accepted monoclonality of RAG-deficient TCR Tg T cells would be due to the suppression of both RAG loci, by cis or trans effects, resulting from the insertion of the neo gene, as reported for other targeted mutations.