Age of Down's Dementia Determined by APP Repeats: The amyloid precursor protein (APP) locus on chromosome 21 influences the development of Alzheimer disease. The relationship between a tetranucleotide repeat on intron 7 of the APP gene and the age at onset of dementia in Down syndrome (DS) causes a 13-year difference in the age at onset of dementia in DS associated with the number of tetranucleotide repeat alleles in APP.  Influence of the amyloid precursor protein locus on dementia in Down syndrome. Margallo-Lana M, Morris CM, Gibson AM, Tan AL, Kay DW, Tyrer SP, Moore BP, Ballard CG. Newcastle, UK. Neurology. 2004 Jun 8;62(11):1996-8