Loss of ATR function and of the related kinase ataxia telangiectasia mutated (ATM) have been linked to defective DNA repair, which has been assumed to cause the genomic instability, including aneuploidy, observed in these disorders and to make ataxia telangiectasia patients prone to cancer (Wright et al., 1998; Spring et al., 2002; Shen et al., 2005; Murga et al., 2009; Lang et al., 2016; Yazinski and Zou, 2016; Blackford and Jackson, 2017; Quek et al., 2017). The gene discussed is ATR; the disease is Ataxia-telangiectasia.