H2AX and ataxia telangiectasia: Such cells exhibit hypersensitivity to ionizing radiation, cell cycle defects and impaired end‐joining during CSR in B cells.48, 49 A homozygous RNF168 mutation was also reported in a patient with a syndrome mimicking ataxia telangiectasia (AT).50 Similar to a previous finding, patient cells displayed defective DNA repair and radiosensitivity due to defective ubiquitination of H2A and H2AX, leading to diminished recruitment of 53BP1 and BRCA1.