When measured in the CSF of individuals with MCI, NfL has been found to be predictive of subsequent progression to AD dementia,4 with a recent meta-analysis showing it to have comparable discriminatory power to the well-established CSF AD biomarkers of Aβ1-42, total tau, and phosphorylated tau.3 Recent studies comparing NfL measurement in CSF and serum have shown close correlation,9,10,22 implying serum NfL may similarly predict subsequent progression, in keeping with our results. This evidence concerns the gene NEFL and Alzheimer disease.