AU-rich elements (AREs) present in the 3'untranslated region of mature lymphokine and cytokine mRNAs regulate mRNA stability and translational efficiency. Based on their sequence features and functional properties AREs can be divided into three classes. Class II AREs direct asynchronous cytoplasmic deadenylation (processive kinetics) generating poly-A(-) mRNAs. Among mRNA with class II ARE in their 3' UTR are GM-CSF, IL-2.The minimum number of AUUUA tandem motifs to activate processive degradation is 4 (of which at least 3 in tandem). Furthermore, an AU-rich region 20-30 nt long immediately 5' to the cluster of AUUUA motifs can greatly enhance the degradation activity.
Chen CY, and Shyu AB AU-rich elements: characterization and importance in mRNA degradation. Trends Biochem Sci 1995; 20(11), 465-70   PubMed