The protein encoded by this gene functions as a cofactor for the stimulation of transcriptional elongation by HIV-1 Tat, which binds to the HIV-1 promoter through Tat-TAR interaction. This protein may also serve as a dual-function factor to couple transcription and splicing and to facilitate their reciprocal activation. Alternatively spliced transcript variants have been found for this gene.[provided by RefSeq, Sep 2009]