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Ubiquitin is a small regulatory protein that is ubiquitous in eukaryotes. Ubiquitination (or Ubiquitylation) refers to the post-translational modification of a protein by the covalent attachment (via an isopeptide bond) of one or more ubiquitin monomers. Ubiquitin (originally, Ubiquitous Immunopoeitic Polypeptide) was first identified in 1975 as an 8.5 kDa protein of unknown function expressed universally in living cells.