Prion protein. Computer artwork of part of a prion protein. Prions cause diseases such as BSE (bovine spongiform encephalopathy) in cows and CJD (Creutzfeldt-Jakob disease) in humans. They are a mutated form of a normal cell protein (PrP). The rod-shaped atoms are colour-coded: hydrogen (light blue), oxygen (red), nitrogen (dark blue) and carbon (orange). Prions consist of amino acids formed into spiralling alpha-helices (pink & orange) and straight beta-sheets (for example the two mauve areas on the strands at lower right). This is part of the rigid and stable half of the protein. The other half (not shown) is variable in shape. One shape is thought to cause CJD and BSE.