The constitutive transport element (CTE) of the type D retroviruses promotes nuclear export of unspliced viral RNAs apparently by recruiting host factor(s) required for export of cellular messenger RNAs. Here, we report the identification of TAP as the cellular factor that specifically binds to wild-type CTE but not to export-deficient CTE mutants. Microinjection experiments performed in Xenopus oocytes demonstrate that TAP directly stimulates CTE-dependent export. Furthermore, TAP overcomes the mRNA export block caused by the presence of saturating amounts of CTE RNA. Thus, TAP, like its yeast homolog Mex67p, is a bona fide mRNA nuclear export mediator. TAP is the second cellular RNA binding protein shown to be directly involved in the export of its target RNA.