The Uq-ruq system is one of the latest discoveries of transposable elements in the early 1980's. The system has been prevalent in assorted maize genetic and breeding materials, suggesting a contribution to maize genome diversity. The Uq-ruq system shares some functional activity with the Ac-Ds system, in that Ac transactivates all Ds elements including Ds1/ruq, while Uq transactivates only Ds1/ruq. Diverse functional aspects of Uq's have been revealed by differential spotting patterns (DSPs) with reporter alleles (a1- or c1-ruq). Probable basis for DSPs can include differential methylation, different positions, and sequence differences among ruq elements;To investigate the nature of the Uq element, a genetic locus containing Uq was characterized. Mn::Uq is a miniature mutant co-segregating with Uq. This mutation affects pollen tube growth and kernel development. Non male-transmission of Mn::Uq necessitates its presence as a heterozygote and a 1:1 segregation of miniature and normal phenotypes. However the ratio was inconsistent, due to a variable penetrance and expressivity in kernel phenotypes. Genetic data indicate that the variable expressivity is attributable to genetic background. Given the inseparability of miniature phenotype from spotting, incomplete female transmission and non-expressivity of Mn::Uq are responsible for the lower penetrance. Genetic mapping identified Mn::Uq as a new gene, locating mn5 on 2S. The co-segregation test of Mn5::Uq with an Ac probe, based on the genetic functional overlapping between the two systems, indicates that Uq is a unique transposon;More than three generations of following Envag with a high transposition rate of ~50% around the a1 locus is an example that a transposon can indefinitely manipulate the maize genome. Question on the transposition mechanism of En/Spm through reciprocal crosses revealed several features, such as epigenetic change in activity, excision timing, relation to plant developmental pattern, and production of progeny phenotypes discordant with parent. Such observations led to the conclusion that the activity of En of a1-m(Au) is not limited to a specific stage or timing during plant development. An En/Spm modifier is found to require two copies for the phenotypic expression.