The protein degradation/ubiquitination pathway involves the regulated post-translational modification of substrate proteins via the addition of ubiquitin. Ubiquitination of substrate proteins signals the protein for degradation by the proteasome. Ubiquitination can also alter protein activity and interactions. The addition of ubiquitin is carried out by specific ubiquitin enzymes. 
Figure 1. Protein ubiquitination is carried out by enzyme complexes such as ubiquitin-E1, ubiquitin-E2, and ubiquitin-E3. Once the protein has been tagged with ubiquitin, it is recognized for degradation by the proteasome. The proteasome degrades the substrate protein into peptide subunits.
Biomarkers in the protein degradation/ubiquitination pathway serve as inclusion eligibility criteria in 33 clinical trials, of which 28 are open and 5 are closed. The genes MDM2 and BTRC on this pathway most frequently harbor alterations that are inclusion eligibility criteria for clinical trials.
Of the trials that contain alteration(s) in the protein degradation/ubiquitination pathway as inclusion criteria, 11 are phase 1 (10 open), 5 are phase 1/phase 2 (3 open), 15 are phase 2 (13 open), and 2 are phase 3 (2 open) .