Ubiquitin is a highly conserved 76 amino acid protein with an estimated molecular weight of 8.56 kDa which has a central role in regulated protein degradation. It is a protein modifier which can be covalently attached to target lysines either as a monomer or as a lysine-linked polymer. Several types of polymeric chains can be formed depending on the lysine used for the assembly. Attachment to proteins as a polymer leads to their degradation by the 26S proteosome; a complex, multicatalytic cytosolic and nuclear protease. Attachment to proteins as a monomer or as an alternatively linked polymer does not lead to proteasomal degradation and may be required for numerous functions, including maintenance of chromatic structure, regulation of gene expression, stress response, ribosome biogenesis and DNA repair. Ubiquitin is synthesized as a polyubiquitin precursor with exact head to tail repeats, the number of repeats of which differ between species and strains. In some species there is a final amino-acid after the last repeat, here in bovine a Cys. Some ubiquitin genes contain a single copy of ubiquitin fused to a ribosomal protein (either L40 or S27a).