Proteasomes are protein complexes inside all eukaryotes and archaea, and in some bacteria which main function is to degrade unneeded or damaged proteins by proteolysis. The 26S proteasome is a eukaryotic ATP-dependent protease (EC 3.4.-) that is known to collaborate with the ubiquitin system, the system that tags proteins with polyubiquitin chains as a marker for protein degradation in eukaryotic cells that degrades ubiquitin conjugates. It consists of no less than 31 principal subunits arranged into two subcomplexes, the 20S core protease (CP) and the 19S regulatory particle (RP). The CP is a broad spectrum ATP- and ubiquitin-independent protease. It is a cylindrical stack created by the assembly of four heptameric rings. The two peripheral rings are composed of seven related α-subunits and the two central rings are composed of seven related β-subunits. The subunits have active sites with various hydrolase cleavage capacities, enabling the 26S proteasome to cleave most, if not all, peptide bonds. Bortezomib (Axon 1810) is a specific inhibitor of 26S proteasome activity with approved application for use in multiple myeloma.