ATR and Ataxia: Nevertheless, considering severe cerebellar atrophy and ataxia in mouse models with deletion of the Nbs1 or Atr in the neuroprogenitors (Nbs1-CNSΔ or Atr-CNSΔ) and the absence of cerebellar deficits of both Nbs1-PCΔ and Mre11-PCΔ mice, neuronal degeneration found in A-TLD (even A-T) patients (Jackson and Bartek, 2009; Chrzanowska et al., 2012), are most likely of a developmental origin of the impaired function of the MRN complex rather than an intrinsic function of this complex in postmitotic Purkinje cells (Stracker and Petrini, 2011; Lee et al., 2012).