To define genetic determinants of tumor cell resistance to the cytotoxic action of tumor necrosis factor α (TNF), we have applied cDNA microarrays to a human breast carcinoma TNF-sensitive MCF7 cell line and its established TNF-resistant clone. Of a total of 5760 samples of cDNA examined, 3.6% were found to be differentially expressed in TNF-resistant 1001 cells as compared with TNF-sensitive MCF7 cells. On the basis of available literature data, the striking finding is the association of some differentially expressed genes involved in the phosphatidylinositol-3-kinase/Akt signaling pathway. More notably, we found that the PRNP gene coding for the cellular prion protein (PrP^c), was 17-fold overexpressed in the 1001 cell line as compared with the MCF7 cell line. This differential expression was confirmed at the cell surface by immunostaining that indicated that PrP^c is overexpressed at both mRNA and protein levels in the TNF-resistant derivative. Using recombinant adenoviruses expressing the human PrP^c, our data demonstrate that PrP^c overexpression converted TNF-sensitive MCF7 cells into TNF-resistant cells, at least in part, by a mechanism involving alteration of cytochrome c release from mitochondria and nuclear condensation.