The Map Viewer provides special browsing capabilities for a subset of organisms in Entrez Genomes. Map Viewer allows you 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. The number and types of available maps vary by organism, and are described in the "data and search tips" file for each organism. 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.

http://www.ncbi.nlm.nih.gov/mapview/