Histone deacetylases are a class of enzymes catalysing the removal of acetyl groups from histones. This allows tighter interactions between the histone protein and the DNA, by restoring the positive charge to the lysine residues found on the histone protein. By strengthening the interactions between the histone and the DNA, transcription is less likely to happen, leading to the silencing of genes wrapped around the histones.
The cell is able to tightly regulate histone deacetylases, allowing it to control the specific regions of DNA under active transcription.