The human genome is under constant invasion by retrotransposable elements. The 
most successful of these are the Alu elements; with a copy number of over a 
million, they occupy about 10 % of the entire genome. Interestingly, the vast 
majority of these Alu insertions are located in gene-rich regions, and one-third 
of all human genes contains an Alu insertion. Alu sequences are often embedded 
in gene sequence encoding pre-mRNAs and mature mRNAs, usually as part of their 
intron or UTRs. Once transcribed, they can regulate gene expression as well as 
increase the number of RNA isoforms expressed in a tissue or a species. They 
also regulate the function of other RNAs, like microRNAs, circular RNAs, and 
potentially long non-coding RNAs. Mechanistically, Alu elements exert their 
effects by influencing diverse processes, such as RNA editing, exonization, and 
RNA processing. In so doing, they have undoubtedly had a profound effect on 
human evolution.