L-Buthionine-(S,R)-sulfoximine is a cell-permeable, potent, fast acting and irreversible inhibitor of g-glutamylcysteine synthetase and depletes cellular glutathione levels. The IC50 value of L-Buthionine-(S,R)-sulfoximine on melanoma, breast and ovarian tumor specimens are 1.9 μM, 8.6 μM, and 29 μM, respectively.