Alu elements are the most abundant repetitive elements in the human genome; they 
have amplified by retrotransposition to reach the present number of more than 
one million copies. Alu elements can be transcribed in two different ways, by 
two independent polymerases. 'Free Alu RNAs' are transcribed by Pol III from 
their own promoter, while 'embedded Alu RNAs' are transcribed by Pol II as part 
of protein- and non-protein-coding RNAs. Recent studies have demonstrated that 
both free and embedded Alu RNAs play a major role in post transcriptional 
regulation of gene expression, for example by affecting protein translation, 
alternative splicing and mRNA stability. These discoveries illustrate how a part 
of the 'junk DNA' content of the human genome has been recruited to important 
functions in regulation of gene expression.