The human PA28 Activator gamma Subunit, also known as 11S REG gamma and Ki antigen, is a 254 amino acid (aa) member of the PA28 family of regulatory proteins with a predicted molecular weight of 29 kDa. The mouse and rat orthologs share 100% aa sequence identity with human PA28 gamma. Two isoforms of PA28 gamma have been identified. Six PA28 gamma subunits combine to form a homo-hexameric ring-shaped PA28 Regulatory Complex. Unlike PA28 Regulatory Complexes composed of PA28 alpha subunits or PA28 alpha/beta subunits, regulatory complexes composed of PA28 gamma subunits have narrow substrate specificity and have been shown to cleave only a few biologically relevant protein substrates. PA28 gamma is involved in cell cycle progression and in T cell-restricted MHC class I presentation of select antigens.