SUMO1,2, and 3 are predominantly located in the nucleus and targets of SUMOylation are predominantly nuclear. Transcription cofactors are nuclear proteins that generally do not bind DNA themselves but interact with DNA-bound factors and influence transcription. SUMOylation of transcription cofactors usually inhibits the activity of the cofactor (reviewed in Girdwood et al. 2004, Gill 2005, Lyst and Stancheva 2007, Garcia-Dominguez and Reyes 2009). In the cases of coactivators such as PPARGC1A (PGC-1alpha) this results in decreased transcription; in the cases of corepressors such as MBD1 this results in increased transcription.