Hyperacetylated Chromatin Domains: Lessons from Heterochromatin*

A small but growing number of loci that exhibit covalent histone modifications, such as hyperacetylation, over broad regions of 10 kb or more have been characterized. These hyperacetylated domains occur exclusively at loci containing highly expressed, tissue-specific genes, and the available evidence suggests that they are involved in the activation of these genes. Although to date little is known concerning the formation or function of these domains, rather more is known concerning repressive, heterochromatic domains, and the example provided by heterochromatin may be instructive in considering mechanisms of active domain formation.