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Acetylation is a chemical reaction that involves the transfer of an acetyl group from one molecule to another. An acetyl group, which has the chemical formula CH3CO–, is commonly derived from acetic acid. This process is a type of acylation. In biological systems, acetylation is a crucial post-translational modification that alters the function of proteins. For instance, acetylation of histones, proteins that package DNA, can change gene expression by affecting how tightly DNA is wound. This modification plays a role in processes like cell division and differentiation.

In organic chemistry, acetylation is often used to protect functional groups or to introduce an acetyl moiety into a molecule. Common acetylating agents include acetic anhydride and acetyl chloride. These reagents react with nucleophilic sites on other molecules, such as hydroxyl groups (alcohols) or amino groups (amines), to form esters or amides, respectively, with the acetyl group attached. This reaction is reversible and can be undone through deacetylation, which removes the acetyl group. The reversible nature of acetylation is important in many biological signaling pathways.