DSIF is a factor that can either negatively or positively affect transcription by RNA polymerase II

RNA polymerase II

RNA polymerase II is an enzyme found in eukaryotic cells. It catalyzes the transcription of DNA to synthesize precursors of mRNA and most snRNA and microRNA. A 550 kDa complex of 12 subunits, RNAP II is the most studied type of RNA polymerase...

 (Pol II). In one case of negative regulation, it can interact with NELF

Negative elongation factor

NELF is a four subunit protein that affects transcription by RNA polymerase II. Subunit NELF-E is also known as RDBP. The NELF-A subunit is encoded by the gene WHSC2, Wolf-Hirschhorn syndrome candidate 2...

 to promote the stalling of Pol II at some genes. This stalling is released by P-TEFb

P-TEFb

Positive Transcription Elongation Factor b is the key factor regulating eukaryotic mRNA transcription at the level of elongation.It is a cyclin dependent kinase, composed of Cdk9 and in humans one of three cyclins, T1, T2, or K....

. In humans, DSIF is composed of hSPT4

SUPT4H1

Transcription elongation factor SPT4 is a protein that in humans is encoded by the SUPT4H1 gene.-Further reading:...

 and hSTP5

SUPT5H

Transcription elongation factor SPT5 is a protein that in humans is encoded by the SUPT5H gene.-Interactions:SUPT5H has been shown to interact with Cyclin-dependent kinase 7, PIN1, CDK9, HTATSF1, POLR2A, PRMT1 and Protein arginine methyltransferase 5....

(SPT4 and SPT5 are homologues in yeast).