The 26S proteasome is a multicatalytic proteinase complex with a highly ordered structure composed of 2 complexes，a 20S core and a 19S regulator. The 20S core is composed of 4 rings of 28 non-identical subunits;2 rings are composed of 7 alpha subunits and 2 rings are composed of 7 beta subunits. The 19S regulator is composed of a base，which contains 6 ATPase subunits and 2 non-ATPase subunits，and a lid，which contains up to 10 non-ATPase subunits. Proteasomes are distributed throughout eukaryotic cells at a high concentration and cleave peptides in an ATP/ubiquitin-dependent process in a non-lysosomal pathway. An essential function of a modified proteasome，the immunoproteasome，is the processing of class I MHC peptides. This gene encodes a protein that inhibits the activation of the proteasome by the 11S and 19S regulators. Alternative transcript variants have been identified for this gene.