The DRB sensitivity-inducing factor (DSIF) complex is a heterodimer that acts as both a positive and negative transcription elongation factor. DSIF together with negative elongation factor (NELF) complex associates with RNA polymerase II and causes transcriptional pausing. Phosphorylation of DSIF-NELF complexes dissociates NELF from the elongation complex and transforms DSIF into a positive elongation factor. (Adapted from FBrf0235113, FBrf0211126 and FBrf0203142).