Polyubiquitin-B(UBB) is one of Ubiquitins. Ubiquitin is one of the most conserved proteins known in eukaryotic organisms. Ubiquitin is required for ATP-dependent, non-lysosomal intracellular protein degradation of abnormal proteins and normal proteins with a rapid turnover. Ubiquitin is covalently bound to proteins to be degraded, and presumably labels these proteins for degradation.