Structure of different types of Ds elements compared with Ac. Ac is 4565-bp long and encodes a 5-exon transposase. Ds1 elements are the shortest and share little in common with Ac. Ds2 elements have ~200 bp of the Ac subterminal region at each end. Ds-l3 elements carry sequences corresponding to parts of exons 2 and 3 of the Ac transposase and are the longest Ds elements, on average. Ds-l4 elements have a modal length of about 1 kb and share with Ac only about 30 bp at either end.