Proteinase-K (PK) sensitivity of the N-terminal (P4) PrPres epitope in goat classical scrapie (CS) samples. A, B PK digestion curves of respective scrapie goat isolates S2 and I5, showing the differential sensitivities to PK of the epitopes of respectively SAF84 (black) and P4 (red). Vertical lines point to the 1 mg mL−1 PK condition used to study a larger selection of CS cases in C. C Shows the sensitivity of samples to PK at 1 mg mL−1 of the N terminal PrPres P4-epitope relative to that of core epitope SAF84. Vertical dashed line separates the samples between the ones with highest (CS-1) and lowest (CS-2) P4 epitope content consistent with data in Additional file 4. The cut-off between the two groups is reflected by the horizontal dash line and is based on the value of the reference sample I11 plus the average value of the standard deviations of all samples (1 + 0.4 = 1.4).