A chromosome anomaly syndrome that emerges with one more dose of 21st chromosome in the number of chromosome, which is 46 in the cell of a normal person. In most of the patients, this extra 21st chromosome appears to be attached to 46 chromosomes and existence of 47 chromosomes comes into question in the patient. In a small part of the patients (~4%), material that belongs to such extra 21st chromosome may be located on a different chromosome (translocation type). This is important both for investigating family transportation and determination of risk of recursion. Patients have mental deficiency. Typical face symptoms and several additional symptoms may be observed in the patients as well as mental deficiency. It is essential to conduct peripheral blood analysis for the person who is considered to have Down syndrome and genetic consultation should be provided upon the results.