Biology is a vast domain. The Mouse Genome Informatics (MGI) system, which focuses on the biology of the laboratory mouse, covers only a small, carefully chosen slice. Nevertheless, we deal with data of immense variety, deep complexity, and exponentially growing volume. Our role as an integration nexus is to add value by combining data sets of diverse types and origins, eliminating redundancy and resolving conflicts. In this paper, we briefly describe some of the issues we face and approaches we have adopted to the integration problem.