Keep the Exits Clear

摘要

Open any cell biology textbook and you will find a transmission electron micrograph of a cross section through a cell nucleus. The dense staining of heterochromatin, which is largely inactive and highly condensed, will be clearly visible as dark patches, often lining the inner surface of the nuclear membrane. However, there will be periodic gaps in these heterochro-matic territories at peripheral sites where the nuclear pore complexes (NPCs) are situated. NPCs are macromolecular structures that enable the transit of nucleic acids and proteins across the nuclear envelope. Although the existence of these NPC-associated heterochromatin-free regions has been known for over 50 years, how they are generated and what their function is have remained unknown.