The essential yeast proteins Yra1 and Sub2 are messenger RNA export factors that have conserved counterparts in metazoans, designated Aly and UAP56, respectively. These factors couple the machineries that function in splicing and export of mRNA. Here we show that both Yra1 and Sub2 are stoichiometrically associated with the heterotetrameric THO complex, which functions in transcription in yeast. We also show that Sub2 and Yra1 interact genetically with all four components of the THO complex (Tho2, Hpr1, Mft1 and Thp2). Moreover, these components operate in the export of bulk poly(A)(+) RNA as well as of mRNA derived from intronless genes. Both Aly and UAP56 associate with human counterparts of the THO complex. Together, these data define a conserved complex, designated the TREX ('transcription/export') complex. The TREX complex is specifically recruited to activated genes during transcription and travels the entire length of the gene with RNA polymerase II. Our data indicate that the TREX complex has a conserved role in coupling transcription to mRNA export.