Full-length cDNAs (fl-cDNAs) are important resources for the characterization of gene function, since they contain all the information required for the production of functional RNAs and proteins. Large sets of fl-cDNA clones have been collected from several plant species and have become available for functional genomic analysis. We have developed a system for the identification of gene function by screening for transgenic plants ectopically expressing fl-cDNAs and named it the FOX (fl-cDNA overexpressor gene) hunting system. This system can be applied to almost all plant species without prior knowledge of their genome sequences because only fl-cDNAs are required. For utilization of the FOX hunting system, Agrobacterium libraries and Arabidopsis seeds carrying rice and Arabidopsis fl-cDNAs are available. Here, we will describe the procedure followed in the FOX hunting system from the generation of expression vectors carrying fl-cDNAs to the confirmation of phenotype in retransformed plants.