PSEN1 and Alzheimer disease: In a cross-sectional study performed on presymptomatic mutation carriers for familial forms of AD, namely carriers of pathogenic mutations in the genes coding for presenilin 1 (PSEN1) and amyloid precursor protein (APP), serum NfL was found to be higher compared to non-carriers, suggesting ongoing quantifiable neurodegeneration already in the presymptomatic phase.