Histone H4 (acetyl K20), or H4K20ac, is a post-translational modification that is generally associated with gene repression in human cells. Contrary to most histone acetylation events that are linked to gene activation, H4K20ac is enriched around the transcription start sites (TSSs) of minimally expressed genes and within the gene body of expressed genes. This unique acetylation mark shows little correlation with known histone modifications, including histone H3 methylations. Transcriptional activators are mostly excluded from genes enriched with H4K20ac, while the transcription repressor NRSF/REST co-localizes with this mark, suggesting that H4K20ac plays a distinct role in the epigenetic regulation of gene expression.