DS is the most common genetic cause of intellectual disability, with an incidence of 1 in 691 live births, or about 6000 of the 4 million infants born yearly in the US.[6] DS occurs in all ethnic groups, socioeconomic levels, and geographical regions. Standard trisomy 21, the presence of an extra chromosome 21, is caused by chromosome non-disjunction and occurs in 95% of cases. In about 4% to 5% of individuals with DS, the trisomy results from a chromosome translocation, and the remaining 1% of cases of DS are mosaic with a combination of typical and trisomic cells.[7]

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