The nuclear pore complex is a huge macromolecular structure (120 MDa in vertebrates). Although individual components of the nuclear pore have been identified at a molecular level, a major challenge in the field is to understand how these components cooperate to direct regulated, bidirectional movement of cargo. Each pore is estimated to guide the transport of more than 100 molecules per minute. Yet, despite the "high throughput" nature of this process, transport occurs with specificity.