One group of genes for which deacetylation has been reported to play a key role is the set of ribosomal RNA (rRNA) genes. These genes are expressed at the Nucleolar Organizer Regions (NORs). Of the Arabidopsis histone deacetylases (HDACs), HDAC6 has been reported as playing a significant role in silencing of the rRNA genes by modifications of those genes . In Figure 1, HDAC6 can be seen localized to the nucleolus where rRNA genes are normally transcribed. In this case HDAC6 switched off the rRNA genes. Early et al.  have also described a model that predicts the key events that allow switching from active transcription to repression of transcription. These key events involve de novo cytosine methylation and histone deacetylation.