<p>RNA polymerase II (<db_xref db="EC" dbkey="2.7.7.6"/>) [<cite idref="PUB00000080"/>, <cite idref="PUB00005357"/>] is one of the three forms of RNA polymerase that exist in eukaryotic nuclei. The C-terminal region of the largest subunit of this oligomeric enzyme consists of the tandem repeat of a conserved heptapeptide [<cite idref="PUB00005359"/>]. The number of repeats varies according to the species (for example there are 17 in Plasmodium, 26 in yeast, 44 in Drosophila, and 52 in mammals). The region containing these repeats is essential for the function of polymerase II. This repeated heptapeptide(called CT7n or CTD) is rich in hydroxyl groups. It probably projects out of the globular catalytic domain and may interact with the acidic activator domains of transcriptional regulatory proteins. It is also known to bind by intercalation to DNA. RNA polymerase II is activated by phosphorylation. The serine and threonine residues in the CT7n repeats are the target of such phosphorylation.</p>