Ataxia-telangiectasia is characterized by radiosensitivity, genome instability and predisposition to cancer 1, 2. Heterozygous carriers of ATM, the gene defective in ataxia-telangiectasia, have a higher than normal risk of developing breast and other cancers 3, 4, 5, 6. We demonstrate here that Atm'knock-in'(Atm-ΔSRI) heterozygous mice harboring an in-frame deletion corresponding to the human 7636del9 mutation show an increased susceptibility to developing tumors. In contrast, no tumors are observed in Atm knockout (Atm+/−) heterozygous mice. In parallel, we report the appearance of tumors in 6 humans from 12 families who are heterozygous for the 7636del9 mutation. Expression of ATM cDNA containing the 7636del9 mutation had a dominant-negative effect in control cells, inhibiting radiation-induced ATM kinase activity in vivo and in vitro. This reduces the survival of these cells after radiation exposure and enhances the level of radiation-induced chromosomal aberrations. These results show for the first time that mouse carriers of a mutated Atm that are capable of expressing Atm have a higher risk of cancer. This finding provides further support for cancer predisposition in human ataxia-telangiectasia carriers.