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Image: The human genome is a complex place, composed of vast amounts of repetitive, retroviral and transposable elements (TEs). Originally thought to be only parasitic DNA elements, TEs substantially outnumber coding sequence genes and take up nearly half of the human genome. Critically, TEs have been attributed a wide range of biological functions, ranging from the direct co-option of viral genes in trophoblast development and the immune system, to evolutionary innovations in transcription factor binding to DNA. TEs are released from repression during early embryogenesis, as the genome is demethylated and reprogrammed, and intriguingly, TEs are similarly released from repression when somatic cells are in vitro reprogrammed to induced pluripotent stem cells. TEs also have intimate links to epigenetic regulation, as TEs are regulated by the epigenetic system, and are simultaneously regulating the epigenetic system. Indeed, it often seems that TEs sit at the center of the crossroads between embryogenesis, embryonic stem cells, reprogramming, and long noncoding RNAs. TEs are fascinating molecules, which sometimes seem to be both parasitic and symbiotic at the same time, and they will likely have many fascinating, as yet undiscovered, important biological roles (see the review by Andrew Paul Hutchins and Duanqing Pei on page 1722-2015 No.20 issue of Science Bulletin).
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