Nab2 shuttles between the nucleus and the cytoplasm and is thought to interact with transcripts following 3′-end cleavage and addition of the poly(A) tail.18,19,22 Thus, Nab2 and other shuttling mRNA-binding proteins could serve as positive markers to target mature transcripts for preferential export from the nucleus. The Nab2 protein (Fig. 1a) has a modular structure based on four functional domains:23 a unique N-terminal domain of approximately 100 residues, a glutamine-rich linker, an RGG domain, and a Zn-finger domain. Deletion analysis indicates that the N-terminal domain is necessary for the nuclear export of both poly(A) RNA and Nab2 in vivo, consistent with a model in which Nab2 associates with mRNA in the nucleus and in which the N-terminal domain facilitates export of both poly(A) mRNA and Nab2.23 The RGG domain, which contains the nuclear localization sequence24 recognized by the nuclear import factor Kap104 (also known as transportin or Kapβ2), is required for the nuclear import of Nab225 to recycle it for another round of mRNA export. The Zn-finger domain is critical for Nab2 binding to polyadenosine mRNA.22,26