Multiple myeloma (MM) is a malignant disorder of differentiated B-cells for which standard care involves the inhibition of the proteasome. Unfortunately, >97% of MM patients develop resistance or become intolerant to current treatments within a few months/years, after which survival is less than one year. We have developed a class of proteasome inhibitors that regulate the proteasome via a mechanistically distinct, non-competitive, protein:ligand interaction that overcomes resistance and is highly effective in vivo. Little is known about how and where non-competitive inhibitors bind to the proteasome or how they exert their biological activity. In this proposal we will utilize a novel mass spectrometry labeling technology to fill this critical void in our understanding of this alternative mechanism and define its differences from the current paradigm of proteasome inhibition.