Recently, we have shown that nitric oxide inhibits DDR signaling (ATM, ataxia telangiectasia mutated and ATR, ataxia telangiectasia and Rad3-related) by a mechanism that is associated with the inhibition of mitochondrial oxidative metabolism and decreases in cellular levels of ATP (39, 41). The gene discussed is ATR; the disease is Ataxia-telangiectasia.