The finding of trisomy 21 mosaicism in AD is particularly interesting because of the universal presence of AD-like pathology and neurodegeneration in full trisomy 21 Down syndrome (DS) patients [8], [29]–[31], and the finding that early onset inherited AD can be caused by a single duplicated APP gene on one chromosome 21 [32], [33]. This evidence concerns the gene APP and Alzheimer disease.