Scientists have taken an important step toward a gene therapy that could one day turn off the extra genetic material that causes Down syndrome (DS). Down syndrome is a genetic condition caused by an extra chromosome 21 (and consequently hundreds of triplicate genes) that leads to developmental and neurological issues. According to the Washington-based National Down Syndrome Society, approximately 1 in every 640 babies in the United States is born with DS. That makes it the most common chromosomal condition.