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. This leads to the additional binding of ubiquitin entities and formation of a polyubiquitin chain (Hofmann RM et al. 1999). Experiments confirming this finding were performed in yeast cells.