"Copy-and-paste" and "cut-and-paste" transposons have been adapted for use as gene transfer vectors. In the top half of the figure, transposition of naturally occurring transposons is depicted. In the lower half of the figure, the general methods used to adapt these transposons for use as gene transfer agents is shown. Direct terminal repeats (TR) flank some retrotransposons. Inverted terminal repeats (IR) flank cut and paste transposons. Retrotransposons, such as the L1 element, encode open reading frames (ORF) of unknown function as well as integrases (IN) and reverse transcriptases (RT). Both kinds of elements can be manipulated so that special vector sequences are inserted. In the case of retrotransposons, the vector sequences are inserted into the 3' untranslated region. In the case of the "cut and paste", DNA transposons, the vector sequences replace the transposase gene, which is expressed from a heterologous promoter in trans.