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. The multi-enzymatic nuclear exosome complex, Rat1p and cap binding complex degrade mRNAs in the yeast nucleus. The mitochondrial degradosome (mtEXO) is an enzymatic complex which has a major function in the mitochondrial genome post-transcription gene expression regulation. RNA interference cleaves mRNA after a previous induction by dsRNA. The 2-5A/RNase L system inhibits replication of viral RNA.