Short interspersed elements (SINEs) are retroelements found in a wide variety of eukaryote genomes. They are 70- to 500-bp repetitive DNA sequences and each has an internal promoter for RNA polymerase III, an A-rich or T-rich tail of various size at the 3′ end, flanked by the direct repeat DNA sequences of several nucleotides, indicating that they have proliferated via retrotransposition. Many SINEs appear to be related to tRNAs, whereas a few, such as the primate Alu and rodent B1 family elements, are related to 7SL RNA, and zebrafish SINE3 is related to 5S rRNA.