RNA degradation in yeast and mammalian cells take place in cytoplasm, nucleus and mitochondria. Degradation of transcripts with a premature termination codon or without stop codon results in repression of aberrant protein synthesis. The mRNA decay pathway, which degrades mRNAs containing a stem-loop structure has been discovered and characterized in the yeast Saccharomyces cerevisiae. Eukaryotic transcripts with AU-rich elements in their 3'-untranslated region have a very short half-life. These elements regulate mRNA level in cells.