PSME1 and neoplasm: The expression levels of PA28α and β have been analyzed in surgically collected human tumor samples and in tumor cell lines of different origin, in the context of a broader evaluation of the expression of other components of the MHC class I antigen processing machinery (APM) (i.e., immunoproteasome catalytic β-subunits, TAP1/2, aminopeptidases, calnexin, calreticulin, tapasin) and HLA class I antigens.