f Cernunnos is a DNA repair factor of the nonhomologous end-joining machinery. Its deficiency in humans causes radiosensitive severe combined immune deficiency (SCID) with microcephaly, characterized in part by a profound lymphopenia. In contrast to the human condition, the immune system of Cernunnos knockout (KO) mice is not overwhelmingly affected. In particular, Cernunnos is dispensable during V(D)J recombination in lymphoid cells. Nevertheless, the viability of thymocytes is reduced in Cernunnos KO mice, owing to the chronic activation of a P53-dependent DNA damage response. This translates into a qualitative alteration of the T cell repertoire to one in which the most distal V␣ and J␣ segments are missing. This results in the contraction of discrete T cell populations, such as invariant natural killer T (iNKT) and mucosa-associated invariant T (MAIT) cells, in both humans and mice.T he immune system is the site of intense genome dynamics, in particular during the development and maturation of B and T lymphocytes in bone marrow and the thymus, when antigen receptor genes are rearranged through V(D)J recombination prior to their expression. DNA damages are also likely to occur during the several phases of intense proliferation which accompany the development of B and T cells.V(D)J recombination is the prototypical example of the generation of a programmed DNA double-strand break (DNA-dsb) during lymphoid development through the activity of recombination activating genes 1 and 2 (Rag1/2) on immunoglobulin (Ig) and T cell receptor (TCR) genes (see the work of Helmink and Sleckman [1] for a recent review). The resulting DNA-dsb is resolved by the nonhomologous end-joining (NHEJ) DNA repair pathway, composed of seven core components (see the work of Lieber [2] for a recent review). The Cernunnos-Xrcc4-DNA ligase IV complex ultimately reseals the DNA-dsb. Cernunnos, also known as Xrcc4-like factor (XLF), was the last NHEJ factor that was independently identified, through a survey of RS-SCID patients (3) and a yeast two-hybrid screen with Xrcc4 as a bait (4). Cernunnos and Xrcc4 adopt the same overall three-dimensional crystal structure (5, 6) and, together with DNA ligase IV, are parts of the same complex (4, 7). Cernunnos stimulates the DNA-joining activity of the Xrcc4-DNA ligase IV complex (8, 9). V(D)J recombination constitutes a central checkpoint in the development of the immune system, as its defect leads to abortive B and T cell maturation in vivo, resulting in severe combined immune deficiency (SCID) (10), but its first recognized function was the generation of a diverse antigenic repertoire through the combinatorial association of variable, diversity, and joining segments that encode the variable domains of both Ig and TCRs (11). Numerous examples show that a reduced V(D)J recombinase activity affects the extent of antigenic diversity of immune receptors in mice and humans. The resulting immune deregulation may then lead to autoimmunity, increased susceptibility to infections, or the development of vario...