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