Down syndrome is a chromosome condition that is associated with intellectual disability, a characteristic facial appearance, and low muscle tone in infancy. The degree of intellectual disability varies from mild to moderate. People with Down syndrome may be born with a variety of health concerns, including heart defects or digestive abnormalities. In addition, they have an increased risk of developing gastroesophageal reflux, celiac disease, hypothyroidism, hearing and vision problems, leukemia, and Alzheimer disease. Down syndrome is caused by having three copies of chromosome 21, instead of the usual two copies. This condition occurs in about 1 in 800 newborns. Researchers have identified genes that cause some of the features of Down syndrome and are working to learn more in order to improve treatment options. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.