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Down Syndrome

Down syndrome is the neurogenetic syndrome most commonly associated with intellectual disability. The incidence is approximately one in every 1,000 live births resulting in 3,000 to 5,000 infants born with Down syndrome internationally each year depending on how researchers account for fetal loss and stillbirths. For unknown reasons, an error in cell division (nondisjunction) causes most cases of Down syndrome, resulting in an individual having three copies of chromosome 21 instead of the typical two copies. The addition of a third chromosome 21 is why the syndrome is also called trisomy 21. Some Down syndrome cases are caused by translocation, when part of chromosome 21 is located in other chromosomes, and mosaicism, when some cells have three copies of chromosome 21.

Most individuals with Down syndrome ...

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