Long interspersed nuclear elements-1 (LINE1 or L1), the only currently active class of retrotransposons in humans, occupy nearly 20% of the genomic real estate (Goodier and Kazazian, 2008). Although ~500,000 copies are present in the genome, only 80–100 are active full length (6 kilobases) elements that can transpose to new genomic locations by a target primed reverse transcription (TPRT) mechanism (Goodier and Kazazian, 2008) (Figure 2D). Both germline and somatic L1 activity contribute significantly to structural variation in human genomes (Lupski, 2010).
