A large complex of proteins found in the cytoplasm of cells that degrades cell proteins into small peptide units. Proteasomes participate in the disposal of damaged proteins and the removal of proteins that have reached the end of their functional life in the cell. They thus play a vital part both in the cell’s response to stress and in the day-to-day running of the cell. A proteasome consists of a hollow cylinder of protease enzymes, each with its catalytic site facing the internal cylinder wall. Another protein complex guards the entrance to the cylinder. Proteins destined for degradation are usually tagged by a chain of small proteins, called ubiquitins, which direct the target protein to unfold and enter the proteasome, where it is cleaved into peptides. Proteasomes are also involved in the production of peptides derived from viruses or other pathogens for presentation by cells in association with MHC class I proteins.