This dissertation by Zhi Guo, supervised by Tanya Paull and other committee members, investigates the activation of the Ataxia-telangiectasia mutated (ATM) protein by oxidative stress. ATM is a key regulator of the cellular response to DNA double-strand breaks (DSBs). The study demonstrates that ATM can be activated by oxidative stress, specifically through exposure to hydrogen peroxide (H₂O₂), independent of DSBs and the Mre11-Rad50-Nbs1 (MRN) complex. In vitro kinase assays and in vivo experiments in 293T cells show that H₂O₂ induces ATM autophosphorylation on serine 1981, leading to the phosphorylation of p53 and Chk2. The critical role of cysteine residue 2991 in ATM activation by H₂O₂ is also highlighted. The research contributes to understanding the multifaceted functions of ATM, including its role in oxidative stress response and DNA damage repair.This dissertation by Zhi Guo, supervised by Tanya Paull and other committee members, investigates the activation of the Ataxia-telangiectasia mutated (ATM) protein by oxidative stress. ATM is a key regulator of the cellular response to DNA double-strand breaks (DSBs). The study demonstrates that ATM can be activated by oxidative stress, specifically through exposure to hydrogen peroxide (H₂O₂), independent of DSBs and the Mre11-Rad50-Nbs1 (MRN) complex. In vitro kinase assays and in vivo experiments in 293T cells show that H₂O₂ induces ATM autophosphorylation on serine 1981, leading to the phosphorylation of p53 and Chk2. The critical role of cysteine residue 2991 in ATM activation by H₂O₂ is also highlighted. The research contributes to understanding the multifaceted functions of ATM, including its role in oxidative stress response and DNA damage repair.