MAPT and Alzheimer disease: The non-linear relationship between tau PET tracer uptake and tau368/t-tau found in a previous study [15], and also observed here, supports that there is a variable processing of tau in healthy subjects, which is then distorted towards secreting a higher relative abundance of N- to mid-terminal tau, being later shifted to produce more C-terminal fragments (which are more prone for aggregation) as AD progresses.