Disclosed are: a method for detecting pathogenic isoform of prion protein as distinguished from normal prion protein in a simple manner, rapidly, with a high degree of sensitivity and quantitatively without the need of the enzymatic treatment with protease K; and a reagent for use in the method. Specifically disclosed are: a pathogenic isoform of prion protein binder which comprises lactoferrin; and a method for detecting pathogenic isoform of prion protein by using the pathogenic isoform of prion protein binder.
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