In the PD subgroup with longitudinal measurements of MMSE (n = 47, follow-up up to 2 years, mean follow-up 0.95 years, SD 0.81), there was a significant interaction between time and baseline NfL concentration, meaning that high baseline plasma NfL concentration was associated with MMSE decline over time (P = 0.020), while adjusting for age, sex, and education. This evidence concerns the gene NEFL and Parkinson disease.