MAPT and Alzheimer disease: Since the small fragments of 3RCF tau lower than 7 kDa were more difficult to digest by PK, we next performed the titration of varied PK concentrations ranging from 0, 1.25 μg/mL, 2.5 μg/mL, 3.75 μg/mL, 5 μg/mL, 7.5 μg/mL, to 10 μg/mL for the 4R (Fig. 8A) and 0, 1.25 μg/mL, 5 μg/mL, 10 μg/mL, 12.5 μg/mL and 25 μg/ml for the 3R tau (Fig. 8B) SAA end-products of skin tau from AD and non-AD subjects.