BACE1 and Alzheimer disease: As PrPC is decreased [17], and BACE1 activity is increased, in sporadic AD [10], [18], and as PrPC negatively modulates BACE1 activity [15], [19], we tested the hypothesis that there is a negative correlation between PrPC and (i) BACE1 activity, (ii) Aβ and (iii) Braak tangle stage, in human brain tissue.