This group represents transcription elongation factor IIS (TFIIS) and uncharacterized transcription factor S-II-related proteins from viruses. TFIIS is a component of RNA polymerase II preinitiation complexes, and is required for preinitiation complex assembly and stability. TFIIS binds to RNA polymerase Pol II, and in stalled and arrested elongation complexes it stimulates the cleavage of the nascent transcript at the 3' end. It enhances the excision of mis-incorporated nucleotides by RNA polymerase II, providing an mRNA proof-reading mechanism during transcription elongation. Three distinct TFIIS isoforms exist in vertebrates.