With a predicted molecular weight of 34 kDa, Tetra-ubiquitin chains are composed of four ubiquitin monomers that are covalently linked through isopeptide bonds, which typically form between a lysine residue of one ubiquitin molecule and the C-terminal glycine residue of another ubiquitin molecule. Each human ubiquitin monomer is 76 amino acids (aa) in length and shares 96% and 100% aa identity with yeast and mouse ubiquitin, respectively. Seven of the 76 aa in ubiquitin are lysine residues that can participate in poly-ubiquitin chain formation. Linkage through specific lysine residues is thought to serve as a signal that affects protein degradation, signaling, trafficking, and other cellular processes. Ubiquitin is not expressed directly as free ubiquitin, but rather as linear fusions either to itself or to certain ribosomal protein subunits. These ubiquitin-fusion precursors are proteolyzed by DUB's at the appropriate junction points to yield active ubiquitin monomers with C-termini ending in GG. There are likely several intracellular DUB's which perform this essential processing role.