Nucleosomes play a dynamic role in transcription. The key to this role is the structure of the flexible and charged histone tails that extend from the hydrophobic nucleosome core. These tails are modified by acetylation and deacetylation, phosphorylation, methylation, ubiquitination and ATP-dependent chromatin remodeling, and they regulate functions as diverse as transcription, DNA-dependent chromatin assembly, DNA repair, mitosis and silencing by heterochromatin.