Purification and Structural Properties of Ubiquitin.- Molecular Genetics of the Ubiquitin System.- Ubiquitin Activation and Ligation.- Ubiquitin/ATP-Dependent Protease.- Ubiquitin Carboxyl-Terminal Hydrolases.- Ubiquitinated Histones and Chromatin.- Immunochemical Probes of Ubiquitin Pool Dynamics.- Protein Breakdown and the Heat-Shock Response.- Lymphocyte Homing Receptors, Ubiquitin, and Cell Surface Proteins.- Role of Transfer RNA in the Degradation of Selective Substrates of the Ubiquitin- and ATP-Dependent Proteolytic System.- The N-End Rule of Selective Protein Turnover.- Selectivity of Ubiquitin-Mediated Protein Breakdown.