For example, the expression of inflammatory cytokine proteins by immune system cells must be finely tuned so that these proteins do not cause more damage to the body than the bacterial infection they were produced to defeat. Excessive inflammation can lead to organ failure and death, and this is exactly what happens when people are stricken with septic shock. Likewise, the loss of control of genetic expression is the underlying cause of various forms of cancer--for instance the overexpression of genes that cause the cell to grow and the suppression of genes that curb cell growth.
Humans and other animals have evolved many overlapping ways of controlling gene expression. Many different controls determine when a DNA gene is transcribed into mRNA, and when and how the mRNA is translated into a protein. Many proteins are also tightly controlled through various post-translational modifications that activate or deactivate them.
Translation is the final step in the expression of every gene, and the cell exercises control over this process in numerous ways, and hence expression of a gene. One way to exert this control is ensuring that mRNAs that are not needed are destroyed. One of the ways that nature has evolved for destroying certain mRNAs is making them inherently unstable and prone to degradation.
This inherent instability is built into the 3' tail of the mRNA in the form of noncoding stretches of RNA called AU-rich elements. Lots of genes, including inflammatory cytokines and cancer-causing oncogenes, have these AU-rich elements built in.
Source:The Scripps Institute