Next to mutations in the amyloid precursor protein (APP) linked with early onset autosomal dominant familial forms of AD, reduction of levels of the disease-linked Aβ42 form of the peptide in the cerebrospinal fluid (CSF) is the earliest known biomarker heralding future AD many years before the first symptoms arise in all known forms of AD (Bateman et al., 2012; Buchhave et al., 2012). This evidence concerns the gene APP and Alzheimer disease.