Prion diseases, such as Creutzfeldt-Jakob disease in humans, bovine spongiform 
encephalopathy in cattle, chronic wasting disease in cervids (i.e., deer, elk, 
moose, and reindeer), and sheep scrapie, are caused by the misfolding of the 
cellular prion protein (PrPC) into a disease-causing conformer (PrPSc). PrPC is 
a normal, GPI-anchored protein that is expressed on the surface of neurons and 
other cell types. The structure of PrPC is well understood, based on studies of 
recombinant PrP, which closely mimics the structure of native PrPC. In contrast, 
PrPSc is prone to aggregate into a variety of quaternary structures, such as 
oligomers, amorphous aggregates, and amyloid fibrils. The propensity of PrPSc to 
assemble into these diverse forms of aggregates is also responsible for our 
limited knowledge about its structure. Then again, the repeating nature of 
certain regular PrPSc aggregates has allowed (lower resolution) insights into 
the structure of the infectious conformer, establishing a four-rung β-solenoid 
structure as a key element of its architecture.