TCEB1 （elongin C） is a subunit of elongin， which is a common transcription elongation factor that increases the RNA polymerase II transcription elongation past template encoded arresting sites. The SIII complex is comprised of elongins A/A2， B and C. The SIII complex activates elongation by RNA polymerase II by curbing transient pausing of the polymerase at numerous sites within transcription units. Elongin A acts as the transcriptionally active component of the SIII complex， while elongins B and C are regulatory subunits. Elongin A2 is specifically expressed in the testis， and able to forming a stable complex with elongins B and C. The VHL （von Hippel-Lindau） tumor suppressor protein binds to elongins B and C， and thus inhibits transcription elongation.
