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) argues against this, but there have been cases reported of incomplete inactivation of ATM with other mutations (Stewart et al., 2001). The fact that both Q3SA and Q1SA cultures have been passaged in our lab for several months argues that if reversion does occur spontaneously, there is unlikely to be a strong functional selection for the reversion, because we have not yet observed a change in apparent genotype.
