The possible involvement of germline mutation of the ataxia telangiectasia mutated (ATM) gene in childhood acute leukemia with mixed lineage leukemia (MLL) gene rearrangement (MLL⁺) was investigated. Of the 7 patients studied, 1 showed a germline missense ATM mutation (8921C>T; Pro2974Leu), located in the phosphatidylinositol-3 (PI-3) kinase domain. In reconstitution assays, the ATM mutant 8921T could only partially rescue the radiosensitive phenotype of AT fibroblasts, and in an in vitro kinase assay, it showed a defective phosphorylation of p53-Ser15. Furthermore, the introduction of 8921T in U2OS cells, characterized by a normal ATM/p53 signal transduction, caused a significant reduction of in vivo p53-Ser15 phosphorylation, suggesting a dominant-negative effect of the mutant ATM over the wild-type protein. Our finding in this patient suggests that altered function of ATM plays some pathogenic roles in the development of MLL⁺ leukemia.