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DNA double strand breaks (DSBs) pose a se-vere threat to genome integrity, but can also be a necessary part of normal cellular processes, such as meiosis and Ig class switch recombina-tion (CSR). Depending on cell cycle phase and DSB structure, different strategies are used for repair. Homologous recombination (HR) de-pends on a homologous DNA template for repair, preferentially the identical sister chromatid, and is therefore mainly active during the S and G2 phases. Nonhomologous end joining (NHEJ), however, is active throughout the cell cycle and is the principal pathway during the G1 phase, when there is no immediate close template for homologous repair. The classical NHEJ path-way requires not only the key components o