Tat may stimulate transcriptional elongation by recruitment of a complex containing Tat-SF1 and a kinase to the human immunodeficiency virus-type 1 (HIV-1) promoter through a Tat-TAR interaction. A complementary DNA for the cellular activity, Tat-SF1, has been isolated. This factor is required for Tat trans-activation and is a substrate of an associated cellular kinase, Cotransfection with the complementary DNA for Tat-SF1 specifically modulates Tat activation. Tat-SF1 contains two RNA recognition motifs and a highly acidic carboxyl-terminal half. It is distantly related to EWS and FUS/TLS, members of a family of putative transcription factors with RNA recognition motifs that are associated with sarcomas.