We did not find a difference between the AD, CAA, and CS groups in age-adjusted analyses for CSF NFL, but in unadjusted analyses, the CAA group had higher NFL than the control participant group, and in the post-hoc analyses, patients with CAA who were amyloid-PET positive had particularly high levels, raising the possibility that this is a useful marker for co-existing AD and CAA pathology. This evidence concerns the gene NEFL and Cowden syndrome 1.