The article discusses the role of ubiquitin-conjugating enzymes (E2s) in the assembly of ubiquitin chains, which are essential for regulating protein localization, activity, and stability. E2s play a crucial role in determining the length, topology, and processivity of ubiquitin chains, as well as the specific linkage of ubiquitin molecules. They interact with ubiquitin-activating enzymes (E1s) and ubiquitin ligases (E3s) to initiate and elongate ubiquitin chains. E2s can switch between chain initiation and elongation, and they regulate the processivity of chain formation to ensure the assembly of long chains. Additionally, E2s can select specific linkage types by non-covalently interacting with the acceptor ubiquitin. The article highlights the importance of E2s in determining the outcome of ubiquitylation and the biological consequences for modified proteins.The article discusses the role of ubiquitin-conjugating enzymes (E2s) in the assembly of ubiquitin chains, which are essential for regulating protein localization, activity, and stability. E2s play a crucial role in determining the length, topology, and processivity of ubiquitin chains, as well as the specific linkage of ubiquitin molecules. They interact with ubiquitin-activating enzymes (E1s) and ubiquitin ligases (E3s) to initiate and elongate ubiquitin chains. E2s can switch between chain initiation and elongation, and they regulate the processivity of chain formation to ensure the assembly of long chains. Additionally, E2s can select specific linkage types by non-covalently interacting with the acceptor ubiquitin. The article highlights the importance of E2s in determining the outcome of ubiquitylation and the biological consequences for modified proteins.