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The “CTD” is the carboxyl- terminal domain of the largest subunit of RNA polymerase II. In humans it is over 350 amino acids long and is comprised of a tandemly repeated 7 amino acid motif whose consensus is tyr-ser-pro-thr-ser-pro-ser. This domain is central to the mechanism by which post-transcriptional events are coupled to transcription. For example, virtually all of the RNA processing machineries possess components that bind to the CTD. How the CTD is arranged in space around the polymerase is still almost completely a mystery. However, the CTD is thought to be quite flexible and may undergo continuous dynamic rearrangements in performing its various functions.