The Map Viewer is a software component of Entrez Genomes that provides special browsing capabilities for a subset of organisms. It allows one to view and search an organism's complete genome, display chromosome maps, and zoom into progressively greater levels of detail, down to the sequence data for a region of interest. If multiple maps are available for a chromosome, it displays them aligned to each other based on shared marker and gene names and, for the sequence maps, based on a common sequence coordinate system. The organisms currently represented in the Map Viewer are listed in the Entrez Map Viewer help document, which provides general information on how to use that tool. The number and types of available maps vary by organism and are described in the “data and search tips” file provided for each organism.