Transport of macromolecules between the nucleus and cytoplasm occurs bi-directionally and is mediated by nuclear pore complexes (NPCs). NPCs are large supermolecular structures that span the nuclear envelope. Although ions, metabolites, and small macromolecules can passively diffuse through the NPC, most larger macromolecules such as proteins and ribonucleoproteins (RNPs) must be transported across the NPC by specific signal-and energy-dependent mechanisms. To date, the import of proteins into the nucleus has been studied in great detail and is considered to be the most well characterized aspect of nucleocytoplasmic transport. Nuclear import is specified by nuclear targeting or nuclear localization sequences (NLSs) in the imported protein.