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Ubiquitination is a post-translational modification where ubiquitin is attached to a substrate protein. This process is carried out in three main steps, activation, conjugation and ligation, performed by ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), and ubiquitin ligases (E3s) respectively. The ubiquitin modification has three general layouts, monoubiquitination, multi-monoubiquitination and polyubiquitination (homotypic or heterotypic). The ubiquitination bonds are always formed with one of the seven lysine residues from the ubiquitin molecule. In a few rare cases nonlysine residues have been identified as ubiquitination targets, such as cysteine, threonine and serine. Ubiquitination can affect proteins in many ways. It can signal for their degradation via the proteasome, alter their cellular location, affect their activity, and promote or prevent protein interactions.