The Alu repeat family comprises short interspersed elements (SINES) present in multiple copies in the genomes of humans and other primates. The Alu sequence is approximately 300 bp in length and is found commonly in introns, 3′ untranslated regions of genes, and intergenic genomic regions. They are mobile elements and are present in the human genome in extremely high copy number. Almost 1 million copies of the Alu sequence are estimated to be present, making it the most abundant mobile element. The Alu sequence is so named because of the presence of a recognition site for the AluI endonuclease in the middle of the Alu sequence. Because of the widespread occurrence of the Alu repeat in the genome, the Alu sequence is used as a universal primer for PCR in animal cell lines; it binds in both forward and reverse directions. The Alu universal primer sequence is as follows: 5′-GTG GAT CAC CTG AGG TCA GGA GTT TC-3′ (26-mer).