IRF-3, a member of the interferon regulatory factor (IRF) family of transcription factors, functions as a molecular switch for antiviral activity. IRF-3 uses an autoinhibitory mechanism to suppress its transactivation potential in uninfected cells, and virus infection induces phosphorylation and activation of IRF-3 to initiate the antiviral responses. The crystal structure of the IRF-3 transactivation domain reveals a unique autoinhibitory mechanism, whereby the IRF association domain and the flanking autoinhibitory elements condense to form a hydrophobic core. The structure suggests that phosphorylation reorganizes the autoinhibitory elements, leading to unmasking of a hydrophobic active site and realignment of the DNA binding domain for transcriptional activation. IRF-3 exhibits marked structural and surface electrostatic potential similarity to the MH2 domain of the Smad protein family and the FHA domain, suggesting a common molecular mechanism of action among this superfamily of signaling mediators.