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, B and C subunit. The BC-box, which mediates binding to the elongin BC complex, has the consensus [APST]-L-x(3)-C-x(3)-[AILV].