SIII, also known as elongin, is a general transcription elongation factor that increases the RNA polymerase II transcription elongation past template-encoded arresting sites. Subunit A is transcriptionally active and its transcription activity is strongly enhanced by binding to the dimeric complex of the SIII regulatory subunits B and C (elongin BC complex). Heterotrimer of an A (A1, A2 or A3), B and C subunit. The C subunit mediates the binding of the elongin BC complex to subunit A. Note: This description may include information from UniProtKB.