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Cover  The nucleus is the defining structure of the eukaryotic cell. This organelle is bound by a double-membrane nuclear envelope, both faces of which are visible in this exquisite freeze-fracture electron micrograph of a rat epitheial cell by Lelio Orci (from Orci and Perrelet, Freeze-Etch Histology, 1975, Springer-Verlag, New York, NY). To facilitate traffic in and out of the nucleus, the envelope is studded with several thousand nuclear pore complexes, the prominent crater-like rings seen here on the envelope. Subsequent work from many labs has yielded a detailed picture of the nuclear pore complex as an eightfold symmetrical structure with a central transporter region. Despite the structural information revealed by such elegant imaging of the nucleus and the nuclear pore, the nucleus remains a fairly mysterious organelle that is only beginning to be understood. While the outer nuclear envelope is in equilibrium with the endoplasmic reticulum, the inner membrane has a distinct and specific protein composition. No membranes are found inside the nucleus, yet it is a highly ordered and dynamic structure, containing nucleoli, multiple types of intranuclear bodies, and specific chromosome territories. All of this organization must be created and maintained through protein-protein and protein-nucleic acid interactions. The nuclear pore complexes have been shown to be stationary within the nuclear envelope (Belgarah et al., J. Cell Biol., 1997; Daigle et al., 2001), most likely through interaction with the nuclear lamina, a fialmentous structure that underlies the envelope. Most striking is the fact that, in higher eukaryotes, the entire complex architecture of the nucleus is completely disassembled and rebuilt with each round of cell division; however, the mechanism by which this occurs is not yet defined. Exploration of these many secrets of the nucleus is one of the most exciting areas of contemporary cell biology.Maureen Powers