MRE11 and Nijmegen breakage syndrome: Similar to cell lines derived from Nijmegen breakage syndrome patients, NBS1 knock-out U2OS cells showed increased radio-sensitivity, displayed defects in G2/M checkpoint activation, DSB resection and ATR activation in response to ionizing radiation (IR), and had MRE11 predominantly localized in the cytoplasm (Supplementary Fig. 5a–f).