Generation and characterization of mutants are important for the investigation of gene function. Gain-of-function technology is one of the most useful approaches for the systematic production of mutant resources. Full-length cDNAs have been collected from various plant species and have become important resources for functional genomics. We have developed a novel gain-of-function technology for the identification of gene function using a full-length cDNA library, and this system has been named as FOX hunting system (Full-length cDNA Over-eXpressing gene hunting system). In this system, full-length cDNAs are randomly expressed in Arabidopsis. We also generated rice FOX Arabidopsis lines in which full-length cDNAs from rice were expressed in Arabidopsis, and we demonstrated that gene function derived from heterologous organisms can be analyzed systematically using the FOX hunting approach. In this protocol, we describe the process of generating Arabidopsis mutants expressing rice full-length cDNA libraries and the methods of identifying genes from the isolated mutants.