Deacetylases remove acetyl groups from protein substrates. In case of histone deacetylases (HDACs), they oppose the effects of histone acetyltransferases (HATs). This results in a compacted, heterochromatin conformation, causing negative gene expression. There are four classes of HDAC enzymes. Classes I, II, and IV contain the classic HDAC enzymes, whereas Class III contains the sirtuin enzymes (with NAD+).