Isotopically labeled ubiquitin is useful in determining total cellular concentrations of ubiquitin, or determining the ratio of free- to substrate-bound ubiquitin using the protein standard absolute quantification (PSAQ) or related methods.  Reductive methylation of lysine residues renders ubiquitin unable to form poly-ubiquitin chains via lysine linkages with other Ub molecules.  Methylated Ub can form an E1-catalyzed active thioester at the C-terminus allowing the molecule to be transferred to the lysines of substrate proteins in a mono-ubiquitinated or multi-mono-ubiquitinated fashion.