The specificity of the 20S proteasome, which degrades many intracellular proteins, is regulated by protein complexes that bind to one or both ends of the cylindrical proteasome structure(1-5). One of these regulatory complexes, the 11S regulator (known as REG or PA28), stimulates proteasome peptidase activity(6,7) and enhances the production of antigenic peptides for presentation by class I molecules of the major histocompatibility complex (MHC)(8,9). The three REG subunits that have been identified REG alpha, REG beta and REG gamma (also known as the Ki antigen).