Proteasome inhibitors (PIs) are currently used in the clinic to treat cancers such as multiple myeloma (MM). However, cancer cells often rapidly develop drug resistance towards PIs due to a compensatory mechanism mediated by nuclear factor erythroid 2 like 1 (NFE2L1) and aspartic protease DNA damage inducible 1 homolog 2 (DDI2). Following DDI2-mediated cleavage, NFE2L1 is able to induce transcription of virtually all proteasome subunit genes . Under normal condition, cleaved NFE2L1 is constantly degraded by proteasome, whereas in the presence of PIs, it accumulates and induces proteasome synthesis which in turn promotes the development of drug resistance towards PIs . Here, we report that Nelfinavir (NFV), an HIV protease inhibitor, can inhibit DDI2 activity directly . Inhibition of DDI2 by NFV effectively blocks NFE2L1 proteolysis and potentiates cytotoxicity of PIs in cancer cells . Recent clinical evidence indicated that NFV can effectively delay the refractory period of MM patients treated with PI-based therapy . Our finding hence provides a specific molecular mechanism for combinatorial therapy using NFV and PIs for treating MM and probably additional cancers.