Transcription factor which plays a key role in defining the composition of the RNA polymerase II (RNAPII) elongation complex and in modulating the production of mature mRNA transcripts. Acts as an assembly factor to recruit various factors to the RNAPII elongation complex and is recruited to the complex via binding to the transcription elongation factor SUPT6H bound to the C-terminal domain (CTD) of the RNAPII subunit RPB1 (POLR2A). The SUPT6H:IWS1:CTD complex recruits mRNA export factors (ALYREF/THOC4, EXOSC10) as well as histone modifying enzymes (such as SETD2) to ensure proper mRNA splicing, efficient mRNA export and elongation-coupled H3K36 methylation, a signature chromatin mark of active transcription. Interacts with SUPT6H; binds preferentially to the POLR2A-bound SUPT6H. Interacts with ALYREF/THOC4, SETD2 and PRMT5. Interacts with HDGFRP2. Interacts (via IBM motif) with PSIP1 (via IBD domain); phosphorylation increases its affinity for PSIP1 (PubMed:29997176). Phosphorylation increases its interaction with PSIP1. Belongs to the IWS1 family. Contaminating sequence. Potential poly-A sequence. Truncated N-terminus.