Thanks to the quantification of neurodegenerative biomarkers (i.e., Aβ peptides, p-Tau and T-Tau), cerebrospinal fluid (CSF) provides a good level of evidence for the in vivo diagnosis of Alzheimer's disease (AD) at the mild cognitive impairment (MCI) stage (1–3). The gene discussed is MAPT; the disease is early-onset autosomal dominant Alzheimer disease.