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.