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Down syndrome, trisomy of chromosome 21, is well investigated because it is a genetic disease with characteristic mental retardation and precocious dementia of Alzheimer type. Maternal serum markers of human chorionic gonadotrophin unconjugated estriol and amyloid precursor protein, nuchal skinfold on ultrasound and new genetic probes are developed to allow better detection of Down syndrome. The overproduction of Aβ42 because of excessive genes is thought to be a leading factor for early onset of dementia in Down syndrome adults. Animal models and transgenic mice may be helpful in determining the specific gene and pathogenesis for mental retardation and precocious dementia.