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. This evidence concerns the gene TP53BP1 and ataxia telangiectasia.