Transposons are ubiquitous mobile genetic elements found in all eu- and prokaryotic cells.
The first transposon identified, the maize Activator element, belongs to the hAT family. The basic structural and functional features of this transposon family and its phylogenetic roots are discussed in detail, including a phylogenetic tree deduced from the amino acid sequence of the most conserved part of the transposon-encoded transposase. It soon became evident that some of these transposons shared close structural and functional characteristics with the Ac element, forming one of the many transposon families. Although the transposon itself has not yet been identified, it appears to be expressed in the human brain, and quite likely represents an active transposable element. The latter element is included in this review, although its coding region has not yet been analyzed.