Inherited mutations in the ataxia-telangiectasia mutated (ATM) gene cause ataxiatelangiectasia(A-T),a rare recessive disorder associated with a high incidence of cancers, more particularly lymphoid malignancies. ATM is a tumor suppressor gene located on chromosome 11q22-23, encoding a large nuclear phosphoprotein playing a key role in the DNA damage response. Recent findings indicate that ATM is also involved in additional cellular processes, like oxidative stress response and metabolic regulation. While ATM alterations have intensely been studied in sporadic adult cancers, our investigations focused on childhood lymphoid malignancies and more recently on neuroblastoma, a rare and often aggressive paediatric cancer. The objective of this thesis is to discuss our publications in the context of the current knowledge concerning A-T and the ATM gene. The role of ATM in DNA damage response and in sporadic cancers is highlighted. The last part of this work focuses on neuroblastoma and our recent findings which suggest that ATM inactivation could play a central role in the progression of this cancer, thus offering potential therapeutic targets.