Ubiquitination is the covalent attachment of ubiquitin molecules to substrate proteins with three enzymatic steps - E1 ubiquitin activation, E2 ubiquitin conjugation and E3 ubiquitin protein ligase. UBE2V1 and UBE2N are E2 ubiquitin-conjugating enzymes that form a complex and bind ubiquitin molecules at K63. Consequently, the E3 ligase PRKN (Parkin) binds this complex and facilitates the addition of more ubiquitin molecules. (Olzmann JA et al. 2007).