Creutzfeldt-Jakob disease (CJD) is the most common form of PrD in humans and is characterised by a spongiform degeneration of the brain accompanied by the accumulation of a misfolded and protease-resistant form of the normal form of prion protein (PrPc), the infectious prion protein isoform (PrPSc) [282]. This evidence concerns the gene PRNP and X-linked retinal dysplasia.