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 priori protein.