May function as a component of the DRB sensitivity-inducing factor complex (DSIF complex), which regulates transcription elongation by RNA polymerase II. Probably enhances transcriptional pausing at sites proximal to the promoter, which may in turn facilitate the assembly of an elongation competent RNA polymerase II complex (By similarity). Interacts with SUPT5H to form the DSIF complex. DSIF interacts with RNA polymerase II and with the positive transcription elongation factor b complex (P-TEFb complex), which is composed of CDK9 and cyclin-T (By similarity). Belongs to the SPT4 family.