Tau and AD pathology commonly coexist in synucleinopathy patients127 and correlate with an acceleration in cognitive decline.128,129 PD patients with lower CSF Aβ42 levels at disease onset also have earlier appearance of cognitive impairment and more rapid conversion to PD-related dementia.68,130,131 The measurement of CSF Aβ42 could, therefore, be of prognostic value by reflecting brain amyloid content even prior to apparent clinical cognitive impairment.132. This evidence concerns the gene MAPT and Alzheimer disease.