The MuDR/Mu transposons of maize are the most aggressive DNA transposons known in terms of their ability to increase the forward mutation frequency by ∼100-fold. Their activities are held in check by multiple host mechanisms that restrict movement to late in development and that minimize the impact on the maize genome. Aspects of MuDR gene expression that are modulated include transcriptional start site choice, alternative intron splicing, and translational activity. Although not all features of MuDR transposase biology are understood, this element has been used to mobilize endogenous Mu elements and genetically engineered Mu transposons to conduct efficient transposon mutagenesis throughout the maize genome. RescueMu, which contains a copy of the pBluescript bacterial plasmid, has been used to sequence maize genes and to generate a large population of tagged alleles for subsequent genetic analysis. Derivatives of this element are in the testing phase for enhancer trap and activation gene tagging in maize.