In this paper, we describe the query interfaces of a practical biological database system - GPCRDB. Distinguishing features of the system include: an embedded smart query engine (for query relaxation), smooth integration of navigation with the more conventional SQL-based query mechanisms, and the top-down style of incremental query result presentation combined with flexible navigation capabilities. Query relaxation is important due to the fact that queries tend not to be expressed exactly by the users, particularly when complex domain knowledge is involved. Navigation capability is desired because it can be an ideal supplement to SQL-based query mechanisms when large, complex data sets are concerned, especially in the WWW environment where hyperlinks are heavily used. Top-down incremental presentation is one of the best ways for a user to conduct the data presentation/retrieval process more reasonably and efficiently toward the point of interest of the user without being lost in (unwanted) details Keywords: Query interface, Database interface, Biological database, Web information system.