In eukaryotic cells, RNAs are transcribed in the nucleus and exported to the cytoplasm through the nuclear pore complex. The RNA molecules that are exported from the nucleus into the cytoplasm include messenger RNAs (mRNAs), ribosomal RNAs (rRNAs), transfer RNAs (tRNAs), small nuclear RNAs (snRNAs), micro RNAs (miRNAs), and viral mRNAs. Each RNA is transported by a specific nuclear export receptor. It is believed that most of the mRNAs are exported by Nxf1 (Mex67 in yeast), whereas rRNAs, snRNAs, and a certain subset of mRNAs are exported in a Crm1/Xpo1-dependent manner. tRNAs and miRNAs are exported by Xpot and Xpo5. However, multiple export receptors are involved in the export of some RNAs, such as 60S ribosomal subunit. In addition to these export receptors, some adapter proteins are required to export RNAs. The RNA export system of eukaryotic cells is also used by several types of RNA virus that depend on the machineries of the host cell in the nucleus for replication of their genome, therefore this review describes the RNA export system of two representative viruses. We also discuss the NPC anchoring-dependent mRNA export factors that directly recruit specific genes to the NPC.

In eukaryotes, gene expression is achieved by four steps: transcription, processing, nuclear export, and translation. Each step requires multiple factors, and frequently two or more pathways are used by a single gene, enabling strictly regulated gene expression. Importantly, eukaryotes, taking advantage of the separated structures of the nucleus and the cytoplasm, have evolved complex and organized mRNA processing mechanisms that permit sophisticated biological activity. The processes are much more complicated than those found in prokaryotes, in which transcription and translation occur linearly in time and place. Here, we review gene expression, focusing on mRNA processing in the nucleus and the gene regulatory systems found at each step. Combination of gene regulation shows the typical phenotype in each cell. Further understanding of the uncertain mechanisms will uncover the gene regulation through mRNA expression.