PrPSc can be generated and propagated from endogenous PrPC following infectious exposure to exogenous PrPSc, while rare inherited forms, such as familial Cruetzfeldt-Jakob disease, fatal familial insomnia and Gerstmann-Sträussler-Scheinker syndrome, result from autosomal dominant mutations in the prion protein gene (PRNP). The gene discussed is PRNP; the disease is fatal familial insomnia.