The different classes of transposable elements and examples of their structures are depicted. LTR, long terminal repeat; Gag, group-specific antigen; Pol, polymerase; Env, envelope protein; UTR, untranslated region; ORF, open reading frame; (A)n, poly(A) tail; A and B, component sequences of the RNA polymerase III promoter; AR, the adenosine-rich segment separating the 7SL monomers; VNTR, variable number target repeats; ITR, inverted terminal repeat.