Eukaryotes contain a highly conserved multi-enzyme system that covalently links ubiquitin to a variety of intracellular proteins that bear degradation signals recognized by this system. The resulting ubiquitin-protein conjugates are degraded by the 26S proteasome, a large ATP-dependent protease. Pathways that involve ubiquitin underlie a multitude of processes, including cell differentiation, the cell cycle and responses to stress.