In addition to containing MREs, the 3′-UTR also often contains AU-rich elements (AREs), which are 50 to 150 bp in length and usually include many copies of the sequence AUUUA. ARE binding proteins (ARE-BPs) bind to AU-rich elements in a manner that is dependent upon tissue type, cell type, timing, cellular localization, and environment. In response to different intracellular and extracellular signals, ARE-BPs can promote mRNA decay, affect mRNA stability, or activate translation. This mechanism of gene regulation is involved in cell growth, cellular differentiation, and adaptation to external stimuli. It therefore acts on transcripts encoding cytokines, growth factors, tumor suppressors, proto-oncogenes, cyclins, enzymes, transcription factors, receptors, and membrane proteins.