The present invention relates to novel uses of AIM3 acting as a tumor suppressor, and more particularly to methods for using an AIM3 protein or a nucleic acid encoding the protein to activate ATM or ATR and to treat ATM- or ATR-mediated diseases. The AIM3 protein according to the present invention interacts directly with ATM/ATR so as to activate ATM/ATR and proteins regulated by ATM/ATR. Also, the AIM3 protein upregulates tumor suppressor gene p53 and its target genes so as to not only inhibit the proliferation of cells but also to induce apoptosis.