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 A2 is transcriptionally active but its transcription activity is not enhanced by binding to the dimeric complex of the SIII regulatory subunits B and C (elongin BC complex). Heterotrimer of an A (ELOA, ELOA2 or ELOA3P), ELOB and ELOC subunit. Specifically expressed in testis. The BC-box, which mediates binding to the elongin BC complex, has the consensus sequence [APST]-L-x(3)-C-x(3)-[AILV].