Alu sequences are frequently encountered during study of human genomic nucleic 
acid and form a major component of repetitive DNA. This review describes the 
origin of Alu sequences and their subsequent amplification and evolution into 
distinct subfamilies. In recent years a number of different functional roles for 
Alu sequences have been described. The multiple influences of Alu sequences on 
RNA polymerase II-mediated gene expression and the presence of Alu sequences in 
RNA polymerase III-generated transcripts are discussed.