Down syndrome is a chromosomal disorder caused by an error in cell division that results in the presence of an additional copy of chromosome 21 (trisomy 21) or additional chromosomal 21 material. Named after John Langdon Down, the first physician to describe the syndrome systematically, Down syndrome is the most frequent chromosomal cause of mild to moderate intellectual disability. It occurs in all ethnic and economic groups. People with the syndrome also are at a greater risk for many other conditions, such as congenital heart disease, hearing loss, leukemia, and dementia and memory loss similar to Alzheimer disease.