PSMB5 and cancer: Finally, there was a strong and statistically highly significant correlation between the ratio of the CPS subunit β5 (PSMB5) to the IPS subunit β5i (PSMB8) and the response to carfilzomib across a spectrum of seven cancer cell lines (Fig. 4E), indicating that the higher the expression of the β5i (PSMB8) subunit, the better the response to carfilzomib in a given tumor.