Ubiquitin is a small polypeptide that can be conjugated via its C-terminus to amine groups of lysine residue on target proteins. This conjunction is referred to as monoubiquitylation. Additional ubiquitin moieties can be subsequently conjugated to this initial ubiquitin, utilizing any one of the seven lysine residues on the surface of ubiquitin. The formation of these ubiquitin chains is referred to as polyubiquitylation. This tag-free recombinant form of human ubiquitin is engineered to have all available lysines mutated to arginines, except at position 63. This molecule, therefore, can only form polyubiquitin chains of K63 linkage type. Covalent attachment of ubiquitin to other proteins serves various functions, but its major role is to t