Advances in DNA sequencing technologies have produced an ever increasing number of sequenced genomes. However, many of the genes identified in these sequencing efforts have unknown functions or functions inferred based upon sequence homology, highlighting the necessity for functional gene analysis. Mutagenesis combined with phenotypic analyses remains a key mechanism for identifying and establishing gene function. Activation tagging is a mutagenic process that uses altered gene expression, usually gene overexpression, to generate mutant phenotypes. We have developed an activation tagging system in barley (Hordeum vulgare L.) based upon a maize (Zea mays L.) transposable element that carries two highly expressed cereal promoters. Insertion of this mobile genetic element in the genome can lead to insertional gene inactivation, gene overexpression and gene silencing through the production of antisense transcripts. This transposable element system has also been introduced into both wheat (Triticum aestivum L.) and maize and transposon mobility observed.