Ubiquitin is a 76 amino acid, highly conserved nuclear and cytoplasmic protein. It is found exclusively in eukaryotes, becomes covalently attached to substrate proteins by enzymes in the Ubiquitin-Proteosome Pathway (UPP) and has a major role in targeting cellular proteins for the ATP-dependent degradation by the 26S proteosome. Ubiquitination also affects proteosome-independent events such as protein localization, activity and function.