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DSIF is a heterodimer consisting of hSPT4 (human homolog of yeast Spt4- p14) and hSPT5 (human homolog of yeast Spt5-p160). DSIF association with Pol II may be enabled by Spt5 binding to Pol II creating a scaffold for NELF binding (Wada et al.,1998). Spt5 subunit of DSIF can be phosphorylated by P-TEFb.