Members of the Tc1/mariner superfamily of transposons isolated from fish appear to be transpositionally inactive due to the accumulation of mutations. Molecular phylogenetic data were used to construct a synthetic transposon, Sleeping Beauty, which could be identical or equivalent to an ancient element that dispersed in fish genomes in part by horizontal transmission between species. A consensus sequence of a transposase gene of the salmonid subfamily of elements was engineered by eliminating the inactivating mutations. Sleeping Beauty transposase binds to the inverted repeats of salmonid transposons in a substrate-specific manner, and it mediates precise cut-and-paste transposition in fish as well as in mouse and human cells. Sleeping Beauty is an active DNA-transposon system from vertebrates for genetic transformation and insertional mutagenesis.