Lymphoid tumors are the most common malignancies in children, and aberrant chromosomal[unreadable] rearrangements are implicated in the generation of many of these cancers. The long term goals of this[unreadable] project remain to understand the mechanisms underlying genomic plasticity and its control in the[unreadable] developing immune system. The variable regions of antigen receptor polypeptide chains are encoded in[unreadable] separate DNA segments that are brought together during lymphoid development by V(D)J[unreadable] recombination, a form of site-specific DNA rearrangement. The proteins RAG-1 and RAG-2 initiate[unreadable] V(D)J recombination by cleaving participating gene segments at specific recombination signal sequences[unreadable] (RSSs). Completion of recombination employs the cellular machinery for non-homologous DNA end[unreadable] joining (NHEJ), a form of DNA double strand break repair (DSBR). V(D)J recombination is coupled to[unreadable] the cell cycle by the periodic destruction of RAG-2. Coordination of V(D)J recombinase activity with[unreadable] NHEJ is required for the maintenance of genomic integrity; when RAG-induced DNA breaks are[unreadable] improperly sensed and repaired, the resulting genomic rearrangements can give rise to lymphoid tumors.[unreadable] Correspondingly, a growing body of evidence indicates that aberrant V(D)J recombination contributes to[unreadable] the pathogenesis of lymphoid malignancies. Renewed funding is requested for continued study of[unreadable] mechanisms that regulate V(D)J recombination and the contributions of these mechanisms to suppression[unreadable] of tumorigenesis. The proposal has the following aims: (1) to define a detailed mechanism by which[unreadable] V(D)J recombination is linked to the cell cycle, by a combination of biochemical analysis and validation[unreadable] in vivo; (2) to understand the physiologic consequences of uncoupling V(D)J recombination from cell[unreadable] cycle, through the use of mice expressing forms of RAG-2 that escape regulation; (3) to elucidate the[unreadable] mechanism by which V(D)J recombination intermediates are disassembled, through development of a[unreadable] cell-free system; and (4) to build a novel, regulable system to study the feedback control of V(D)J[unreadable] recombination, employing chemical rescue of mutant kinase activites in mouse B cell progenitors.