**4. Conclusions**

DSB repair through NHEJ has been considered rather simple reaction, basically comprised of six core factors, Ku70, Ku80, DNA-PKcs, XRCC4, DNA Ligase IV and XLF. However, the mechanism how these molecules are recruited to DSBs and assembled into repair machinery is not fully understood. It has been difficult even to observe the recruitement of NHEJ molecules by immunofluorescence or simple labeling with fluorescent proteins. However, laser microirradiation technique combined with fluorescent protein and biochemical fractionation enabled us to capture the binding of NHEJ factors to DSBs. NHEJ would involve a number of processing enzymes, whose function or regulation is largely unclear. Additionally, most recent study shed light on the importance of chromatin remodeling prior to the binding of Ku. Obviously, further studies are warranted to elucidate this complexity.
