Repair of DNA double-strand breaks by the nonhomologous end joining pathway is central for proper development of the adaptive immune system. This repair pathway involves eight factors, including XRCC4-like factor (XLF)/Cernunnos and the paralog of XRCC4 and XLF, PAXX nonhomologous end joining factor (PAXX). Xlf−/− and Paxx−/− mice are viable and exhibit only a mild immunophenotype. However, mice lacking both PAXX and XLF are embryonic lethal because postmitotic neurons undergo massive apoptosis in embryos.
