The invention relates to a method for isolating nucleic acid molecules which are capable of specific interaction with native PrP.sup.Sc and can differentiate between native PrP.sup.c and native PrP.sup.Sc. The inventive method comprises the following steps: incubating a pool of nucleic acids with purified PrP.sup.Sc preparations (PPPs); selecting and isolating the resulting protein/nucleic acid complexes and optionally amplifying the specific nucleic acids, in addition to optionally repeating the incubation and isolation stage. The invention also relates to the nucleic acid molecule produced by this method and the use thereof for diagnosing and treating transmissible spongiform encephalopathies.