Duplication of APP alone, in the absence of chromosome 21 trisomy, is a cause of early onset Alzheimer’s disease (Rovelet-Lecrux et al., 2006; Sleegers et al., 2006) making it likely that three copies of APP are important in the development of Down syndrome–Alzheimer’s disease. The gene discussed is APP; the disease is early-onset autosomal dominant Alzheimer disease.