IRFs constitute a family of transcription factors involved in IFN signaling and in the development and differentiation of the immune system. IRFs activities are regulated at transcriptional level (such as IRF1) and post-translational modifications (such as IRF3 and IRF7). Here we show that IRF2 interacts with the SUMO-E3 ligase PIASy and is sumoylated in vivo. Mutagenesis analysis suggests that IRF2 contains three sumoylation sites. Sumoylation of IRF2 has no significant effects on its nuclear localization and DNA-binding activity, but increases its ability to inhibit IRF1 transcriptional activity and decreases its ability to activate the ISRE and H4 promoters. Our findings suggest that sumoylation of IRF2 regulates its transcriptional activities. (c) 2008 Elsevier Inc. All rights reserved.