Acetylation is a form of histone modification in which an acetyl group is added to lysine residues on the N-terminal tail of histone proteins. Acetylation adds a negative charge to the histones. These negative charges repel negatively charged DNA, resulting in a more loosely coiled chromatin structure. The relaxed chromatin conformation significantly increases gene expression by allowing the binding of transcription factors. 

Acetylation is involved in several cellular processes including cell proliferation, cell regulation, and apoptosis.