Transcription factor that binds RNA polymerase II and inhibits the elongation of transcripts from target promoters. Regulates transcription elongation in a TATA box-dependent manner. Necessary for TAT-dependent activation of the human immunodeficiency virus type 1 (HIV-1) promoter. Binds formin (By similarity). Interacts (via the second WW domain) with TREX1 (via proline-rich region) (By similarity). Binds RNA polymerase II, HD and SF1. Detected in brain neurons. Up-regulated in brain tissue from patients with Huntington disease. The FF domains bind the phosphorylated C-terminus of the largest subunit of RNA polymerase II, probably mediate interaction with HTATSF1 and preferentially bind peptides with the consensus sequence [DE](2-5)-[FWY]-[DE](2-5). The WW domains bind Pro-rich domains. Extended N-terminus.