TNF and breast carcinoma: To test the observation that PrPC rescues MCF-7 human breast carcinoma cells from TNF-α-mediated cell death [31], we first determined the sensitivity of MCF-7 cells to TNF-α treatment using two different assays: MTT dye reduction, and flow cytometry after propidium iodide staining to measure the proportion of cells with sub-2n DNA content.