Although the deficient repair of DNA double-strand breaks and X-ray sensitivity are primarily associated with defects in ataxia telangiectasia mutated (ATM), ataxia telangiectasia (A-T) patients are defective in deoxycytosine salvage [10], and exhibit defective mitochondria [11], and cerebellar degeneration [12,13]. The gene discussed is ATM; the disease is cerebellar degeneration.