The functional architecture of the eukaryotic cell nucleus is of great interest to cell biologists. The identification of nuclear proteins — especially nucleic-acid–binding proteins (e.g., transcription factors) — is important for an understanding of genome regulation and function, and provides clues about the molecular function of novel proteins. The nucleus contains a cell’s genetic information and is the site of gene expression. Biological processes involving nucleic acids, such as transcription, replication, recombination, and DNA repair all involve the action of proteins that bind nucleic acids in a sequence-specific manner. These proteins interact with other proteins, which may or may not bind directly to nucleic acids. As a result of these interactions, large functional complexes are formed and anchored to specific nucleic acid sites. Many auxiliary proteins, enzymes, and complexes in the nucleus have important, general functions and are present in small to moderate amounts. However, proteins that bind at selected sites, such as transcription factors bound to specific promoters, represent only 0.01–0.001% of total cellular protein.