Mammalian double-strand break (DSB) repair. DNA DSBs are predominantly repaired by either non-homologous end-joining (NHEJ) or homologous recombination (HR) . NHEJ rejoins broken DNA ends, and often requires trimming of DNA before ligation can occur. This can lead to loss of genetic information. In NHEJ, the broken DNA ends are bound by the KU70/KU80 heterodimer, which orchestrates the activity of other repair factors and recruits the phosphatidylinositol 3-kinase DNA-PKcs/PRKDC. DNA-PKcs phosphorylates and activates additional repair proteins, including itself and the ARTEMIS/DCLRE1C nuclease. ARTEMIS and/or the heterotrimeric MRE11-RAD50-NBN complex are thought to process the DNA ends prior to ligation. The DNA ends are joined by the activity of polymerases and a ligase complex consisting of XRCC4, XLF/NHEJ1 and LIG4. In contrast to NHEJ, HR is an error-free repair pathway that utilizes a sister chromatid, present only in the S- or G2-cell cycle phase, as template to repair DSBs. HR is initiated by DNA end-resection, involving the MRE11-RAD50-NBN complex and several accessory factors including nucleases. The MRE11-RAD50-NBN complex also recruits the phosphatidylinositol 3-kinase ATM, which phosphorylates histone H2AX and many other proteins involved in repair and checkpoint signaling. Single-stranded DNA generated by DNA end-resection is bound by RPA, which is subsequently replaced by RAD51. RAD51 promotes the invasion of the single-stranded DNA to a homologous double-stranded DNA template, leading to synapsis, novel DNA synthesis, strand dissolution, and repair. Many more proteins are involved in both NHEJ and HR, which are not depicted here for clarity, as they are not referred to in the main text. For details, see recent reviews by Lieber  and San Filippo et al. .