The human tumor suppressor gene ataxia telangiectasia mutated (ATM) encodes a 3056 amino-acid protein kinase that regulates cell cycle checkpoints. ATM is defective in the neurodegenerative and cancer predisposition syndrome ataxia-telangiectasia. ATM protein kinase is activated by DNA damage and responds by phosphorylating downstream effectors involved in cell cycle arrest and DNA repair, such as p53, MDM2, CHEK2, BRCA1 and H2AX. ATM is probably a component of, or in close proximity to, the double-stranded DNA break-sensing machinery. We have observed purified human ATM protein, ATM-DNA and ATM-DNA-avidin bound complexes by single-particle electron microscopy and obtained three-dimensional reconstructions which show that ATM is composed of two main domains comprising a head and an arm. DNA binding to ATM induces a large conformational movement of the arm-like domain. Taken together, these three structures suggest that ATM is capable of interacting with DNA, using its arm to clamp around the double helix.