Types of transposable element. DNA transposons such as the Mariner-like elements in mammalian genomes are inactive relics of mobile DNAs that transpose directly. They encode a transposase activity that mediates their excision from, and integration into, DNA. Retrotransposons transpose via an RNA intermediate that is converted by a reverse transcriptase (RT) into a DNA that is integrated into the host-cell genome. Retrotransposons that contain many (but not all) of the activities necessary for their mobility are called autonomous. They comprise the endogenous retroviruses (ERVs), which are characterized by (a) long terminal repeats (LTRs) and (b) the LINEs, which are non-LTR retrotransposons. Of the LINES, only the L1 group is known to be actively mobile in eutherian mammals. R2 is a site-specific element that is active in insects and is the element studied by Eickbush and Eickbush . Non-autonomous elements, such as Alu and SVAs, are dependent on L1 for their mobility. Processed pseudogenes derive from spliced RNAs that are copied and inserted in the genome by the L1s. Mariner: DR, direct repeat (in the host-cell DNA); ITR, inverted terminal repeat. ERV: TSD, target-site duplication (in the host cell DNA); LTR, long terminal repeat; Pol, RNA-dependent DNA polymerase with a reverse transcriptase (RT) domain and an integrase (INT) domain; Env, Gag and Prt encode other proteins required for the virus life cycle. LINE: EN, endonuclease domain; RT, reverse transcriptase domain; C, zinc knuckle domain; DB, DNA binding; An, poly(A). SINE: A/B, A- and B-box PolIII promoter. Reproduced with permission from .