MAPT and early-onset autosomal dominant Alzheimer disease: In the related field of Alzheimer’s disease biomarkers, the availability of definitive markers of disease pathology such as Positron emission tomography (PET) scans and CSF markers of both amyloid and tau pathology have enabled such studies, e.g. PET Amyloid can be used as an endpoint or outcome variable for biomarker discovery.13,14 However, such designs are not yet possible in Parkinson’s disease—there is no PET radioligand or CSF assay of alpha-synuclein to act as the equivalent anchor point.