Ubiquitin-protein ligases (E3s) determine the substrate specificity of ubiquitylation and, until recently, had been classified into two families, the HECT and RING-finger families. The U-box is a domain of similar to70 amino acids that is present in proteins from yeast to humans. The prototype U-box protein, yeast Ufd2, was identified as a ubiquitin chain assembly factor (E4) that cooperates with a ubiquitin-activating enzyme (E1), a ubiquitin-conjugating enzyme (E2), and an E3 to catalyze the formation of a ubiquitin chain on artificial substrates. We recently showed that mammalian U-box proteins, in conjunction with an El and an E2, mediate polyubiquitylation in the absence of a HECT type or RING-finger type E3. U-box proteins have thus been defined as a third family of E3s. We here review recent progress in the characterization of U-box proteins and of their role in the quality control system that underlies the cellular stress response to the intracellular accumulation of abnormal proteins. (C) 2003 Elsevier Science (USA). All rights reserved.