In the letter by Tian et al. (Nature Cell Biology 11; 211–218, 2009), the reported sequence of the synthetic 16mer phosphopeptide used to generate the phospho-specific antibody against ATM Ser794 in the Methods section 'Generation of anti-phospho-Ser794 ATM antibody' contained an error. The correct sequence should be acetyl-CLSNATKKpSPNKIASG-amide.