TP53 and Ataxia-telangiectasia: And finally, regarding the role of p53 in the regulation of S-phase, there does not seem to be a great number of studies directly addressing the issue.19 Radiation induced delays in the S-phase have been described and their regulation has been analysed since Painter and Young discovered radioresistant DNA synthesis in Ataxia telangiectasia cells in 198020, but it was not until a decade and a half later that authors began to speak of an S-phase damage checkpoint.21,22 This checkpoint seemed to be independent of p53.