A typical eukaryotic genome harbors a rich variety of repetitive elements. The most abundant are retrotransposons, mobile retroelements that utilize reverse transcriptase and an RNA intermediate to relocate to a new location within the cellular genomes. A vast majority of the repetitive mammalian genome content has originated from the retrotransposition of SINE (100-300 bp short interspersed nuclear elements that are derived from the structural 7SL RNA or tRNA), LINE (7kb long interspersed nuclear element), and LTR (2-3 kb long terminal repeats) transposable element superfamilies. Broadly labeled as "evolutionary junkyard" or "fossils", this enigmatic "dark matter" of the genome possesses many yet to be discovered properties.