Human prion diseases occur in inherited, sporadic and acquired forms. The inherited forms are associated with coding mutations in the prion protein gene and the identification of one of these pathogenic mutations allows definitive diagnosis and has resulted in a widening of the previously recognized phenotypic spectrum of these diseases. Study of acquired prion disease provides evidence for genetic susceptibility to development of disease following treatment with contaminated pituitary hormones. Sporadic prion disease occurs predominantly in individuals homozygous with respect to a common PrP polymorphism at residue 129. The identification of pathogenic PrP alleles and the role of the codon 129 PrP gene polymorphism in determining susceptibility to prion disease provides strong support for the idea that an abnormal isoform of PrP, PrPSc, is the principal constituent of the prion and that its propagation involves direct PrP-PrP interactions which occur most readily between identical PrP molecules.