Prion diseases or transmissible spongiform encephalopathies (TSEs) are manifested as genetic, infectious or sporadic, lethal neurodegenerative disorders involving alterations of the prion protein (PrP). Infectious PrPSc is highly expressed in the brain of animals affected by TSEs, including scrapie in sheep, BSE in cattle, and Cruetzfeldt-Jacob disease in humans. The PRND gene locus, located on human chromosome 20p, encodes for the doppel protein (Dpl), which exhibits approximately 25% sequence homology with PrP. Dpl is characterized by an alpha-helical conformation, intramolecular disulfide bonds, and two N-linked oligosaccharides, and it is presented on the cell surface by a glycosylphosphatidylinositol anchor. Dpl is highly expressed in adult testis and heart and is detectable in the brain of neonatal mice. Dpl does not appear to contribute to prion disease progression, but ectopic expression of Dpl is implicated in neuronal degeneration of ataxic PRP-deficient mice. Dpl is also thought to play a role in angiogenesis, specifically maturation of the blood-brain barrier.