Transmissible spongiform encephalopathies (TSEs) are caused by a unique infectious agent which appears to be identical with PrPSc, an oligomeric, misfolded isoform of the cellular prion protein, PrPC. All inherited forms of human TSEs, i.e., familial Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, and fatal familial insomnia, segregate with specific point mutations or insertions in the gene coding for human PrP. Here we have tested the hypothesis that these mutations destabilize PrPC and thus facilitate its conversion into PrPSc. Eight of the disease-specific amino acid replacements are located in the C-terminal domain of PrPC, PrP(121-231), which constitutes the only part of PrPC with a defined tertiary structure. Introduction of all these replacements into PrP(121-231) yielded variants with the same spectroscopic characteristics as wild-type PrP(121-231) and similar to full-length PrP(23-231), which excludes the possibility that the exchanges a priori induce a PrPSc-like conformation. The thermodynamic stabilities of the variants do not correlate with specific disease phenotypes. Five of the amino acid replacements destabilize PrP(121-231), but the other variants have the same stability as the wild-type protein. These data suggest that destabilization of PrPC is neither a general mechanism underlying the formation of PrPSc nor the basis of disease phenotypes in inherited human TSEs.