PRNP and scrapie: The presence in solution of the β-PrP23–231 monomer and a distinct, well defined β-PrP23–231 oligomeric species was confirmed by asymmetric-flow field-flow fractionation (AF4), a technique that has previously been used to separate small non-fibrillar β-sheet-rich oligomers of PrP from scrapie-infected mouse brain material (12).