Ubiquitination involves at least three classes of enzymes: ubiquitin-activating enzymes (UBE1s), ubiquitin-conjugating enzymes (UBE2s), and ubiquitin-protein ligases (UBE3s). When ubiquitin is activated by a UBE1, it is transferred to the cysteine residue on a UBE2. UBE2 then binds a UBE3, which transfers the ubiquitin from the UBE2 cysteine to a lysine residue on the target protein. Ubiquitin-conjugating enzyme E2 Q1 (UBE2Q1), also known as ubiquitin-protein ligase Q1 or ubiquitin carrier protein Q1, is an 422 amino acid protein belonging to the ubiquitin-conjugating enzyme (UBE2) family. Two named isoforms of UBE2Q1 exist as a result of alternative splicing events.