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. 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.
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