Fig. 1  Structures of transposable elements. These elements could be categorized into retrotransposons (Alu, long interspersed element [LINE], and human endogenous retrovirus [HERV]) (A) and DNA transposons (e.g., MARINER) (B) based on their manner of mobilization. In addition, autonomous elements (e.g., HERV and LINE) have coding genes responsible for their own mobilization but also other nonautonomous elements (e.g., Alu and short interspersed element/variable number of tandem repeats/Alu [SVA]). Alu consists of two monomers separated by an A-rich connector, one of which, the left monomer, includes internal RNA polymerase III promoter (A and B boxes). Full-length of LINE is ~6 kb and has open reading frames (ORFs) encoding RNA-binding protein, endonuclease, and reverse-transcriptase, which are flanked by untraslated regions (UTRs). ORF1 and ORF2 are separated by an ~60-bp-long intergenic spacer (IS). SVA contains a (CCCTCT)n hexamer, Alu-like sequences, variable number of tandem repeat (VNTR), and short interspersed element-R (SINE-R). An arrow on Alu-like sequences indicates the direction of Alu. HERV has gag, prt, pol, and env genes flanked by a long terminal repeat (LTR), which encodes capsid protein, protease, polymerase, and envelop protein, respectively, used in viral infection. As an example of DNA transposon, mariner has a gene encoding transposase with a DNA-binding domain and catalytic domain flanked by an inverted repeat (IR). All elements are flanked by target site duplication (TSD) through integration. DDE, the conserved DDE sequence of the mariner transposase; NLS, nuclear localization signal.