© Cambridge University Press 2014. The regulated biosynthesis, folding, trafficking, and degradation of proteins are essential for normal cell growth. Central to processes that regulate protein homeostasis are the ubiquitin-proteasome system (UPS) and related ubiquitin-like (UBL) protein conjugation systems, dysregulation of which is associated with neoplastic transformation and cancer (1–5). Ubiquitin conjugation is a reversible post-translational modification, whereby the C-terminal Gly residue of ubiquitin is covalently linked to a primary amino group in a target protein. As shown in Figure 40.1, ubiquitin is a small, 76-amino-acid protein that shares a characteristic ubiquitin-fold structure with a growing number of related UBLs, which include SUMO, NEDD8, ISG15. The post-translational modification of hundreds of proteins by ubiquitin or UBLs has profound effects on all cellular processes. Thus, malfunctions in the UPS and UBL conjugation pathways are emerging as critical factors in the development of human disease, including cancer. Here we provide a brief overview of the UPS and then focus on recent advances in the therapeutic targeting of these pathways. The ubiquitin proteasome pathway Ubiquitin conjugation (or ubiquitylation) is accomplished by an ATP-dependent cascade of enzymatic activities (Figure 40.2; 1,3,6–8). Ubiquitin is first synthesized as an inactive precursor that is cleaved by a ubiquitin isopeptidase to yield the mature di-Gly C-terminus. The mature form of ubiquitin is then adenylated by the ubiquitin-activating enzyme (UAE or E1), to form a high-energy thioester linkage with the E1 active-site Cys. In a transthiolation step, ubiquitin is then transferred to the active site Cys of an Ub-conjugating enzyme or E2. The E2-ubiquitin complex then binds to an E3 ligase enzyme and substrate to effect the transfer of ubiquitin to the acceptor Lys in the substrate protein. As described below, multiple rounds of ubiquitylation can produce chains, which affects a wide range of cellular processes. The de-ubiquitylating enzymes (DUBs) catalyze the removal of ubiquitin from protein substrates, as well as the recycling and editing of ubiquitin chains. A similar enzymatic cascade is involved in the selective conjugation of UBLs, as in the SUMOylation and NEDDylation pathways. For the most part, the E1, E2, and E3 constituents are selective for ubiquitin or a specific UBL. However, some exceptions to this rule and the functional cross-talk of ubiquitin, NEDD8, and SUMO are highlighted below.