The Mouse Tumor Biology database (MTB) has been developed to provide electronic access to mouse tumor biology data. The primary focus of MTB is spontaneous and induced tumors in genetically defined mice (inbreds, hybrids, mutants and genetically engineered strains). By making such data readily available online, MTB provides the scientific community with a much-needed, easily accessible central resource for rapidly finding and evaluating the expanding volume of mouse tumor data. MTB provides cancer researchers with access to data on mouse models for cancer and includes information such as tumor names and classifications, pathology reports, histopathological images, genetics of the strain, genomic changes in the tumor, strain names, tumor frequency and latency, references and links to other related online resources. The MTB database is designed to facilitate the selection of experimental models for cancer research, the evaluation of mouse genetic models of human disease, the review of patterns of mutations in specific cancers and the identification of genes that are commonly mutated across a spectrum of cancers. Recent enhancements to MTB include a redesigned Web interface for more powerful querying and improved views that simplify and better organize data presentations. A significantly expanded archive of histopathological images also is available. The MTB prototype database is accessible at http://tumor.informatics.jax.org. User support is available by e-mail to mgi-help@informatics.jax.org.