TP53 and Ataxia-telangiectasia: T cells from Ataxia-Telangiectasia (AT) and Nijmegen Breakage Syndrome (NBS) patients are resistant to the p53-dependent apoptosis induced by DNA intercalating agents like Actinomycin D [33] and display a clear p53 accumulation defect following genotoxic stimuli such as ionizing radiation (IR) (Figure 4A).