Ubiquitin/proteasome-dependent proteolysis plays an essential role in degrading regulatory proteins and thereby controlling processes of cell proliferation and cell death (apoptosis). Most recent experiments using cell cultures and mouse models have demonstrated that proteasome inhibitors induce cancer cell apoptosis and therefore inhibit tumor growth. The proteasome inhibitors have the following unique features: (i) greater apoptosis-inducing potency when tested in various human tumor cell lines than current anticancer drugs; (ii) ability to selectively target transformed and tumor, but not normal, human cells; and (iii) ability to overcome tumor cell resistance to cytotoxic therapies. We suggest that proteasome inhibitors have potential use as novel anticancer agents. Copyright 1999 Harcourt Publishers Ltd.