Mutations in exon 12 of the nucleophosmin (NPM1) gene (NPMc+ (NPM1 COOH terminal mutations)) define a distinct subset of acute myelogenous leukemias (AMLs), in which the NPMc+ protein localizes aberrantly to the leukemic cell cytoplasm. We have found that introduction of the most common NPMc+ variant into K562 and 32D cells sensitizes these cells to apoptosis induced by drugs such as bortezomib and arsenic trioxide (ATO) that induce reactive oxygen species (ROS) formation, and that cytotoxicity is prevented in the presence of N-acetyl-L-cysteine (NAC), an ROS scavenger. The substitution of tryptophan 288 (W288) by cysteine occurs in the great majority of NPM1c+ mutations. Mutagenesis of cysteine 288 to alanine re-localizes NPMc+ from the cytoplasm to the nucleolus and attenuates the sensitivity of cells expressing this mutation to bortezomib and ATO. Primary AML cells expressing NPMc+ are also significantly more sensitive than other AML cells to apoptosis induced by both drugs at pharmacologically achievable doses. We conclude that the presence of a cysteine moiety at position 288 results in the cytoplasmic localization of NPM1c+ and the increased sensitivity to bortezomib and ATO. These data suggest that bortezomib and ATO may have increased therapeutic efficacy in NPM1c+ leukemias.