Abstract: We present a general model for data integration systems using a mediated schema to represent commonalities in the underlying sources. These sources are mapped to the mediated schema using source descriptions. Users can pose queries against the mediated schema, allowing the system to generate automatically a query plan that enumerates and ranks all possible ways in which the query could be answered. We apply this approach to the domain of online genetic databases, demonstrating the system's ability to answer relevant queries across multiple sources.