Down syndrome is a genetic condition that occurs in 1 out of every 691 live births. It affects people of all ages, races and economic levels and is the most frequently occurring chromosomal disorder. It occurs when there are three, rather than the usual two, copies of chromosome 21 in every cell of the body. Instead of 46 chromosomes, a person with Down syndrome has 47. It is this additional genetic material that alters the course of development and causes the characteristics associated with the syndrome. Down syndrome affects more than 350,000 people in the United States alone. Recent advances in our understanding of Down syndrome have resulted in dramatic improvements in the life span and potential of those who are affected.