Genomic DNA is being sequenced and annotated at a rapid rate, with terabases of DNA currently deposited in GenBank and other repositories. Genome browsers provide an essential collection of resources to visualize and analyze chromosomal DNA. The University of California, Santa Cruz (UCSC) Genome Browser provides annotations from the level of single nucleotides to whole chromosomes for four dozen metazoan and other species. The Genome Browser may be used to address a wide range of problems in bioinformatics (e.g., sequence analysis), comparative genomics, and evolution.