Ubiquitin is the founding member of a family of structurally conserved proteins that regulate
a host of processes in eukaryotic cells. Ubiquitin and its relatives carry out their functions
through covalent attachment to other cellular proteins, thereby changing the stability,
localization, or activity of the target protein. This article reviews the basic biochemistry of
these protein conjugation reactions, focusing on ubiquitin itself and emphasizing recent
insights into mechanism and specificity.