Prion diseases are thought to be caused by the conversion of the normal, or cellular, prion protein (PrP^C) into an abnormal protease-resistant conformation (PrP^res). There are three familial forms of human prion disease, Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler-Scheinker syndrome, and fatal familial insomnia (FFI) which are all expressed at advanced age despite the congenital presence of the mutant prion protein (PrP^M). The cellular mechanisms that result in the age-dependent conversion of PrP^M into PrP^res and the unique phenotypes associated with each PrP^M are unknown. FFI and a familial type of Creutzfeldt-Jakob disease (CJD¹⁷⁸), share the D178N mutation in the PrP gene but have distinct phenotypes linked to codon 129, the site of a methionine/valine polymorphism (129M/V). We analyzed PrP processing in cells transfected with constructs reproducing the FFI and CJD¹⁷⁸ genotypes. The D178N mutation results in instability of the mutant PrP which is partially corrected by N-glycosylation. Hence, only the glycosylated forms of PrP^M reach the cell surface whereas the unglycosylated form is degraded. The unglycosylated PrP^M is also under-represented in the brain of FFI patients validating the cell model. These results offer new insight into the effect of the D178N mutation on the metabolism of the prion protein.