MAPT and proteostasis deficiencies: Interestingly, this work suggests that longitudinal p-tau217 can also be accurately modelled, but unlike PET proteinopathy tracers that measure insoluble protein aggregates directly in the tissue, p-tau217 assays measure an N-terminal soluble form of tau that may serve as an indirect biofluid-based indicator (a reporter) of C-terminal tau aggregation into insoluble tangles in the tissue (the higher the p-tau217 concentration, the more intraneuronal C-terminal tau that can be the substrate of tau aggregation).