An attempt to induce reversion with XR-induced mutagenesis revealed isolated cases of cells staining positive for both pATM and γH2A.X in Q3SA cultures that should carry two mutated alleles to produce only truncated proteins. These small numbers of apparently reverted cells did not rise to the level of significance when assessed by automated counting of all available wells, so one might argue that these spontaneous events occur at an exceedingly low level, if at all, in the absence of mutagenesis. On the other hand, we found pATM and γH2A.X staining cells in Q1SA, which expresses a missense mutation from one allele. These cells, harboring a variant encoding S2394L, could potentially have destroyed a site of autophosphorylation, but a previous search did not identify this serine residue as phosphorylated (Kozlov et al., 2011). On the basis of the possibility that this variant was “leaky” for ATM function, we could not distinguish if actual gene reversion had occurred or if ATM had been activated in only a small proportion of cells. The absence of γH2A.X staining without the mutagenic XR (Figure 1F)
