While loss of the DNA damage–responsive protein kinase ATM (causal for A-T) or the DNA base excision repair factor APTX (aprataxin, a nucleotide hydrolase, mutated in AOA1) results in severe spinocerebellar ataxia, mouse models of these diseases fail to show overt neurodegeneration or behavioral phenotypes. Here, APTX is linked to cerebellar ataxia.