Short interspersed elements (SINEs) are repetitive DNA sequences invading the genomes of eukaryotes. They can amount to more than 10% of the genome in some species but are missing from other ones. While these genomic parasites can be deleterious to the cell, the long term being in the genome has made SINEs a valuable factor of genetic variation providing regulatory elements for gene expression, alternative splice sites, polyadenylation signals, and even functional RNA genes.