16q24 is frequently deleted in multiple tumors including cancers of nasopharynx, esophagus, breast, prostate and liver. By array comparative genomic hybridization (aCGH), we refined a 16q24 hemizygous deletion in nasopharyngeal carcinoma (NPC) cell lines. Semi-quantitative RT-PCR analysis revealed interferon regulatory factor 8 (IRF8) as the only downregulated gene within this deletion. IRF8 belongs to a family of interferon (IFN) regulatory factors that modulate various important physiologic processes including host defense, cell growth and differentiation and immune regulation. In contrast to the broad expression of IRF8 in normal adult and fetal tissues, transcriptional silencing and promoter methylation of IRF8 were frequently detected in multiple carcinoma (except for hepatocellular) cell lines (100% in NPC, 88% in esophageal and 18-78% in other carcinoma cell lines) and in a large collection of primary carcinomas (78% in NPC, 36-71% in other carcinomas). Methylation of the IRF8 promoter led to the disruption of its response to IFN-gamma stimulation. Pharmacological and genetic demethylation could restore IRF8 expression, indicating a direct epigenetic mechanism. Ectopic expression of IRF8 in tumor cells lacking its expression strongly inhibited their clonogenicity, confirming its tumor suppressor function. Thus, IRF8 was identified as a functional tumor suppressor, which is frequently silenced by epigenetic mechanism in multiple carcinomas.