Component of the DRB sensitivity-inducing factor complex (DSIF complex), which regulates transcription elongation by RNA polymerase II. DSIF enhances transcriptional pausing at sites proximal to the promoter, which may facilitate the assembly of an elongation competent RNA polymerase II complex. DSIF may also promote transcriptional elongation within coding regions. DSIF is required for the transcriptional induction of heat shock response genes and regulation of genes which control anterior-posterior patterning during embryonic development (By similarity). Interacts with Spt5 to form DSIF. DSIF interacts with TRX, RNA polymerase II and with the FACT complex, which is composed of DRE4/SPT16 and SSRP/SSRP1. DSIF can also interact with the exosome, a complex with 3'-5' exoribonuclease activity which is composed of at least Csl4, Dis3, Mtr3, Rrp4, Rrp6, Rrp40, Rrp42, Rrp46 and Ski6. DSIF may also interact with the positive transcription elongation factor b complex (P-TEFb complex), which is composed of Cdk9 and cyclin-T (CycT). Belongs to the SPT4 family.