Classical nonhomologous end joining (C-NHEJ) is a major mammalian DNA double-strand break (DSB) repair pathway. Core C-NHEJ factors, such as XRCC4, are required for joining DSB intermediates of the G1 phase-specific V(D)J recombination reaction in progenitor lymphocytes. Core factors also contribute to joining DSBs in cycling mature B-lineage cells, including DSBs generated during antibody class switch recombination (CSR) and DSBs generated by ionizing radiation. The XRCC4-like-factor (XLF) C-NHEJ protein is dispensable for V(D)J recombination in normal cells, but because of functional redundancy, it is absolutely required for this process in cells deficient for the ataxia telangiectasia-mutated (ATM) DSB response factor. The recently identified paralogue of XRCC4 and XLF (PAXX) factor has homology to these two proteins and variably contributes to ionizing radiation-induced DSB repair in human and chicken cells. We now report that PAXX is dispensable for joining V(D)J recombination DSBs in G1-arrested mouse pro-B-cell lines, dispensable for joining CSR-associated DSBs in a cycling mouse B-cell line, and dispensable for normal ionizing radiation resistance in both G1-arrested and cycling pro-B lines. However, we find that combined deficiency for PAXX and XLF in G1-arrested pro-B lines abrogates DSB joining during V(D)J recombination and sensitizes the cells to ionizing radiation exposure. Thus, PAXX provides core C-NHEJ factor-associated functions in the absence of XLF and vice versa in G1-arrested pro-B-cell lines. Finally, we also find that PAXX deficiency has no impact on V(D)J recombination DSB joining in ATM-deficient pro-B lines. We discuss implications of these findings with respect to potential PAXX and XLF functions in C-NHEJ.T o repair DNA double-strand breaks (DSBs), mammalian cells use two major DSB repair pathways: homologous recombination (HR) and classical nonhomologous end joining (C-NHEJ). HR requires a long homologous template for repair and is active in S/G2 cell cycle phase (1). In contrast, C-NHEJ, which directly ligates DSBs with short (1-4 bp) or no homologies, functions throughout interphase, including during the G1 cell cycle phase (2-4). C-NHEJ repairs diverse DSBs, including those arising ectopically by ionizing radiation as well as DSBs generated as intermediates during V(D)J recombination in developing progenitor (pro)-B and -T lymphocytes and Ig heavy chain (IgH) class switch recombination (CSR) in activated mature B lymphocytes (5). The C-NHEJ pathway has a number of characterized members, including "core" C-NHEJ factors that were so-named in part based on their requirement for joining known types of ends via C-NHEJ and their evolutionary conservation (6). Such core factors include the Ku70 and Ku80 proteins, which form the "Ku" end recognition complex, and the XRCC4 and DNA Ligase4 proteins, which form the C-NHEJ ligation complex (5, 6).Exons that encode Ig and T-cell receptor (TCR) variable region exons are assembled from germline V, D, and J gene segments. This V(D)J recombinatio...