To assess whether in MPM, as in the paradigmatic PIs-sensitive cancer MM, enhanced susceptibility towards cytotoxic effects of bortezomib correlates with lower overall potential proteasomal proteolytic capacity, we measured the three main peptidase activities of 26S proteasomes in cellular extracts from the four MPM clones by specific fluorogenic substrates in the presence of ATP. Here, LAP3 is linked to cancer.