MAPT and early-onset autosomal dominant Alzheimer disease: Cerebrospinal fluid (CSF) levels of the 3 established (classical) biomarkers for Alzheimer’s disease (AD), namely amyloid peptide β with 42 amino acids (Aβ42), tau protein phosphorylated at a threonine residue at position 181 (τP-181) and total tau protein (τT) are the gold standard of fluid-based diagnosis of AD [1,2].