Abstract：The Nobel Prize in Chemistry 2004 is shared among three scientists who have madefundamental discoveries concerning how cells regulate the breakdown of intracellular proteins with extreme specificity as to target, time and space. Aaron Ciechanover, Avram Hershko and Irwin Rose together discovered ubiquitin-mediated proteolysis, a process where an enzyme system tags unwanted proteins with many molecules of the 76-amino acid residue protein ubiquitin. The tagged proteins are then transported to the proteasome, a large multisubunit protease complex, where they are degraded. Numerous cellular processes regulated by ubiquitin-mediated proteolysis include the cell cycle, DNA repair and transcription, protein quality control and the immune response. Defects in this proteolysis have a causal role in many human diseases, including a variety of cancers.