To investigate if the revealed differences in gene expression correlate with the amount of proteasome subunits in cancer cells, we analyzed the relative levels of mRNAs encoding the immunoproteasome subunits PSMB8, PSMB9 and PSMB10 in four breast cancer cell lines: MCF-7, SKBR3, ZR-75-1 and BT-474. The gene discussed is PSMB9; the disease is cancer.