SLC6A3 and Cognitive impairment: Indeed, there is evidence that dopaminergic axonal degeneration is an early feature of Parkinson disease.50,51 In the present cohort, we previously found that high NfL in early Parkinson disease correlates with reduced striatal DAT activity and with motor disease severity, especially in bradykinesia and gait measures.35 The relation between high NfL and cognitive deficits shown here is also compatible with a recent cross-sectional study.52 The most accurate biomarker for the prediction of PDD was the combination of NfL with Aβ42 in CSF.