APP and early-onset autosomal dominant Alzheimer disease: People with DS may develop symptoms of Alzheimer's disease (AD) earlier in life than others because of the increased production of the amyloid precursor protein (APP), the precursor peptide of beta-amyloid located on chromosome 21. Increased protein expression of genes on chromosome 21 causes a cascade of effects on the fetal brain's structural development that impacts behavior throughout the lives of people with DS [4].