Histone modifications alter DNA accessibility. Histone modifications alter DNA accessibility. DNA in the condensed heterochromatin state is tightly wound around the histone core. The tight coiling prevents other DNA binding proteins, such as RNA polymerase or transcription factors, from accessing the DNA. In the euchromatin state the histones are less closely associated with DNA and transcription is able to occur. The change between euchromatin and heterochromatin is a dynamic process regulated by modifications to the histone proteins. These modifications include acetylation, methylation and phosphorylation. TF: transcription factor; RNA pol: RNA polymerase. Andrew L. Durham, and Ian M. Adcock Breathe 2013;9:278-288 ©2013 by European Respiratory Society