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Down syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21, also known as trisomy 21.

But Down syndrome — also known as trisomy 21 — is a remarkably variable disease.

It was the first time scientists had found a link between an intellectual disability and a chromosomal anomaly; the condition is now known as trisomy 21.

In a study described last month in Cell Reports, his group showed it prevented otherwise-lethal immune hypersensitivity in mice with trisomy 21.

It's typical to have 23 instruction manuals, each in duplicate, but people with trisomy 21 have an extra, third, copy of manual 21.