PRNP and prion disease: In the absence of PK digestion, CDI is ineffective at discriminating between prion disease and non-prion disease specimens: using the mean D-N value for non-CJD cases plus 3 s.d. as a cut-off value (158 μg PrP per gram brain) only sCJD VV2 subtype, vCJD and GSS (type 1 PrPres) give values above this cut-off.