Characterizing gene function is one of the major challenging tasks in the postgenomic era. To address this challenge, we developed GeneFAS (gene function annotation system), a computer system with a graphical user interface for cellular function prediction by integrating information from protein-protein interactions, protein complexes, microarray gene expression profiles, and annotations of known proteins. GeneFAS can provide biologists a workspace for their organism of interest, to integrate different types of experimental data and annotation information, and facilitate biological discovery and hypothesis generation using all the information. It also provides testing and training capabilities for users to utilize and integrate their data more efficiently. GeneFAS is freely available for download at http://digbio.missouri.edu/genefas .