Studies on hereditary CJD and FFI have contributed greatly to the understanding of all forms of prion disease. Most importantly, they have provided strong support for the prion hypothesis [2]. The linkage of pathogenic PRNP mutations to human prion disease strengthens the notion that a change in PrP conformation is a key event that triggers the development of the disease. Although hereditary CJD and FFI account for only 10% of all cases of human prion disease, they provide a unique opportunity for studying disease pathogenesis initiated by perturbation in the PrP structure. An understanding of the events that accompany a change in PrP conformation has far-reaching implications for sCJD (the most common form of the disease) and for sporadic fatal insomnia. A wealth of available evidence indicates that a common pathway in disease pathogenesis may be shared by both the sporadic and the hereditary forms of prion disease, except that the initiating events are stochastic in the former, rather than predetermined by the presence of a germ-line mutation. In addition, investigations of hereditary CJD and FFI have provided plausible mechanisms of phenotypic heterogeneity in prion disease, a phenomenon analogous to the "prion strain" diversity in animal prion disease. Although many other neurodegenerative diseases such as Alzheimer's disease, amyotrophic lateral sclerosis, and Huntington's chorea are fairly homogeneous in disease phenotype, prion disease includes many clinically and pathologically distinct disease entities. In hereditary prion disease, the disease phenotype is likely to be determined by the combined effect of pathogenic mutations, codon 129 polymorphism, and the type of PrPSc. The pathogenic mutations include point mutations that are located mostly in the central and C-terminal region of PrP, and deletion and insertion mutations that are located in the N-terminal region. It is conceivable that these distinct types of mutations may result in differential changes in conformation or stability of PrP. The codon 129 polymorphism plays a twofold role in modulating the disease outcome. On the mutant allele, it determines the basic features of the disease phenotype--as in the case of FFI and CJD178--that result respectively from the coupling of M or V at codon 129 with the D178N mutation. On the normal allele, it may modulate the severity of the phenotype. A PrPSc subtype is encoded by the PRNP haplotype, and subsequently is generated by a conformational conversion process that transforms the cellular isoform to the pathogenic protein. The site for the formation of a specific PrPSc conformer and its accumulation in different brain regions are likely to contribute to the clinical features and pathologic lesions. The phenotypic homogeneity in other neurologic diseases, including Alzheimer's disease, may be due, in part, to the lack of a powerful genetic modifier such as the codon 129 polymorphism in the PrP gene, and the lack of the ability of affected gene products such as PrP to assume multiple protein conformations. Clearly, the remaining issue in the understanding of pathogenesis of prion disease is a detailed and accurate knowledge of the in vivo processes and conditions for the formation of PrPSc that inevitably lead to the development and expression of the disease. This knowledge will enable the development of a rational and effective strategy for therapeutic intervention.