Prion diseases are unusual neurodegenerative disorders that can be both infectious and inherited. Both forms are hypothesized to result from a posttranslational structural alteration in the cell surface glycoprotein PrPc (cellular isoform of the prion protein) that converts it into the protease-resistant isoform PrPSc (scrapie isoform of the prion protein). However, a direct comparison of molecular events underlying these two manifestations of prion diseases has not been possible, because there has been no cell culture model for the familial forms. We report here that when mutant prion proteins associated with three different inherited prion disorders of humans are expressed as their murine homologues in cultured Chinese hamster ovary cells, the proteins are protease-resistant and detergent-insoluble, two biochemical properties characteristic of infectious PrPSc. In addition, each mutant protein remains tightly associated with the plasma membrane after enzymatic cleavage of its glycosylphosphatidylinositol anchor, a property that we now show is also typical of infectious PrPSc. The cell culture system described here is the first in vitro model for familial prion diseases and provides compelling evidence that infectious and genetic cases share common molecular features.