SUMO modiWcation is reported to inXuence the assembly of transcription factors on promoters and the recruitment of chromatin-modifying enzymes, and is often associated with transcriptional repression (Gill 2004) . Recently, the colocalization of SUMO-1 and UBF in the GFC (Casafont et al. 2007 ) of neuronal cells and the nucle-olar localization of the sentrin/SUMO-speciWc proteases, SENP3 and SENP5 (Gong and Yeh 2006; Nishida et al. 2000) suggest a potential role of sumoylation on the regulation of rDNA transcription. Further studies of the identiWcation of sumoylated nucleolar transcription factors will be necessary to verify this possibility.