H3K9ac is an epigenetic modification to the DNA packaging protein Histone H3. It is a mark that indicates the acetylation at the 9th lysine residue of the histone H3 protein. The H3K9 histone has two jobs. Genes get turned on if this mark is acetylated and silences them if methylated. H3K9ac is an important acetylation and connected with active promoters. H3K9ac and H3K14ac have been shown to be part of the active promoter state. They are also present over bivalent promoters and active enhancers. This is also a mark for liver cancer through a defect in the H3K9ac/H3K9me3 transition.