Ubiquitin (Ub) is an 8 kDa  highly conserved polypeptide, commonly expressed in eukaryotic cells. Ub is added to lysine residues of the target proteins. This post-translational modification (known as ubiquitination) is made through the sequential action of 3 enzymes (E1 Ub activating enzyme, E2 conjugating enzyme and E3 ligase).