In gene expression, DSIF (DRB Sensitivity Inducing Factor) is a protein that can either negatively or positively affect transcription by RNA polymerase II (Pol II). In one case of negative regulation, it can interact with negative elongation factor (NELF) to promote the stalling of Pol II at some genes. This stalling is relieved by P-TEFb. In humans, DSIF is composed of hSPT4 and hSPT5 (SPT4 and SPT5 are homologs in yeast).