IFN-inducible proteins are known to mediate IFN-directed antitumor effects. The human IFN-inducible protein absent in melanoma 2 (AIM2) gene encodes a 39-kDa protein, which contains a 200-amino-acid repeat as a signature of HIN-200 family (hematopoietic IFN-inducible nuclear proteins). Although AIM2 is known to inhibit fibroblast cell growth in vitro, its antitumor activity has not been shown. Here, we showed that AIM2 expression suppressed the proliferation and tumorigenicity of human breast cancer cells, and that AIM2 gene therapy inhibited mammary tumor growth in an orthotopic tumor model. We further showed that AIM2 significantly increased sub-G(1) phase cell population, indicating that AIM2 could induce tumor cell apoptosis. Moreover, AIM2 expression greatly suppressed nuclear factor-kappaB transcriptional activity and desensitized tumor necrosis factor-alpha-mediated nuclear factor-kappaB activation. Together, these results suggest that AIM2 associates with tumor suppression activity and may serve as a potential therapeutic gene for future development of AIM2-based gene therapy for human breast cancer.