an acidic nuclear protein with a carboxy-terminal repeat; contains seven MAP kinase sites; homologous to yeast chromatin structural protein SPT5; forms a complex with SPT4 and is a subunit of DSIFwhich controls RNA polymerase II elongation; RefSeq NM_003169 (human); NM_009296 (Spt4 mouse); NM_013676 (Spt5 mouse)