Health Encyclopedia



Down Syndrome

National Organization for Rare Disorders, Inc.


It is possible that the main title of the report Down Syndrome is not the name you expected. Please check the synonyms listing to find the alternate name(s) and disorder subdivision(s) covered by this report.


  • chromosome 21, mosaic Down syndrome
  • chromosome 21, translocation Down syndrome
  • trisomy 21

Disorder Subdivisions

  • None

General Discussion

Down syndrome is a chromosomal condition in which all or a portion of chromosome 21 appears three times (trisomy) rather than twice in cells of the body. In some individuals with this condition, only a percentage of cells may contain the chromosomal condition (mosaicism).

Symptoms and findings may vary greatly in range and degree, depending on the specific length and location of the duplicated (trisomic) portion of chromosome 21 as well as the percentage of cells containing the condition. However, in many individuals with this condition, such characteristics may include low muscle tone (hypotonia); a tendency to keep the mouth open with protrusion of the tongue; and distinctive facial characteristics including a short, small head (microbrachycephaly), upwardly slanting eyelid folds (palpebral fissures), a depressed nasal bridge, a small nose, and a relatively flat facial profile. Individuals with Down syndrome may also have unusually small, misshapen (dysplastic) ears; a narrow roof of the mouth (palate); vertical skin folds covering the inner corners of the eyes (epicanthal folds); dental abnormalities; and excessive skin on the back of the neck. Other characteristics include unusually short arms and legs; short fingers; and unusual skin ridge patterns (dermatoglyphics) on the fingers, palms, and toes. Individuals with Down syndrome may also have short stature, poor coordination, mild to severe intellectual disability, and hearing impairment.

In some cases, Down syndrome may also be characterized by structural malformations of the heart at birth (congenital heart defects). In addition, those with the condition may have an increased susceptibility to respiratory disease (e.g., pneumonia), other infectious diseases, and malignancies in which there is an increased proliferation of certain white blood cells (leukemia). Rarely do these conditions lead to potentially life-threatening complications.


March of Dimes Birth Defects Foundation

1275 Mamaroneck Avenue

White Plains, NY 10605

Tel: (914)997-4488

Fax: (914)997-4763

Tel: (888)663-4637



The Arc

1825 K Street NW, Suite 1200

Washington, DC 20006

Tel: (202)534-3700

Fax: (202)534-3731

Tel: (800)433-5255

TDD: (817)277-0553



National Down Syndrome Congress

30 Mansell Ct

Suite 108

Roswell, GA 30076


Tel: (770)604-9500

Fax: (770)604-9898

Tel: (800)232-6372



National Down Syndrome Society

666 Broadway

8th Floor

New York, NY 10012-2317


Fax: (212)979-2873

Tel: (800)221-4602



PHP - Parents Helping Parents, Inc.

Sobrato Center For Nonprofits San Jose

1400 Parkmoor Avenue,

Suite 100

Santa Clara, CA 95126


Tel: (408)727-5775

Fax: (408)286-1116

Tel: (866)747-4040



Association for Children with Down Syndrome, Inc.

4 Fern Place

Plainview, NY 11803

Tel: (516)933-4700

Fax: (516)933-9524



NIH/National Institute of Child Health and Human Development

31 Center Dr

Building 31, Room 2A32


Bethesda, MD 20892

Fax: (866)760-5947

Tel: (800)370-2943

TDD: (888)320-6942



Birth Defect Research for Children, Inc.

976 Lake Baldwin Lane

Orlando, FL 32814


Tel: (407)895-0802



International Mosaic Down Syndrome Association

PO Box 1052

Franklin, TX 77856

Tel: (979)828-4177

Fax: (775)295-9373

Tel: (888)637-5465



R.O.C.K. (Raising Our Celiac Kids)

3527 Fortuna Ranch Rd

Encinitas, CA 92024


Tel: (858)395-5421

Fax: (858)756-0431



New Horizons Un-Limited, Inc.

811 East Wisconsin Ave

P.O. Box 510034

Milwaukee, WI 53203


Tel: (414)299-0124

Fax: (414)347-1977



Genetic and Rare Diseases (GARD) Information Center

PO Box 8126

Gaithersburg, MD 20898-8126

Tel: (301)251-4925

Fax: (301)251-4911

Tel: (888)205-2311

TDD: (888)205-3223


Madisons Foundation

PO Box 241956

Los Angeles, CA 90024

Tel: (310)264-0826

Fax: (310)264-4766



ANDO (Apoyo al Nino Down)

Dept. of Medical Genetics, Children's National Medical Center

111 Michigan Avenue

Suite 1950

Washington, DC 20010

Tel: (202)476-5793

Fax: (202)476-2390


National Foundation for Celiac Awareness

P.O. Box 544

224 South Maple Street

Ambler, PA 19002

Tel: (215)325-1306

Fax: (215)643-1707



Perkins School for the Blind

175 North Beacon Street

Watertown, MA 02472

Tel: (617)924-3434

Fax: (617)926-2027



National Consortium on Deaf-Blindness

The Teaching Research Institute

345 N. Monmouth Avenue

Monmouth, OR 97361

Tel: (800)438-9376

Fax: (503)838-8150

Tel: (800)438-9376

TDD: (800)854-7013



PLAN Institute for Caring Citizenship

Suite 260 - 3665 Kingsway


British Columbia, V5R 5W2


Tel: 6044399566

Fax: 6044397001



Child Neurology Foundation

2000 West 98th Street

Bloomington, MN 55431


Tel: (952)641-6100

Fax: (952)881-6276

Tel: (877)263-5430



Medical Home Portal

Dept. of Pediatrics

University of Utah

P.O. Box 581289

Salt Lake City, UT 84158

Tel: (801)587-9978

Fax: (801)581-3899



For a Complete Report

This is an abstract of a report from the National Organization for Rare Disorders (NORD). A copy of the complete report can be downloaded free from the NORD website for registered users. The complete report contains additional information including symptoms, causes, affected population, related disorders, standard and investigational therapies (if available), and references from medical literature. For a full-text version of this topic, go to and click on Rare Disease Database under "Rare Disease Information".

The information provided in this report is not intended for diagnostic purposes. It is provided for informational purposes only. NORD recommends that affected individuals seek the advice or counsel of their own personal physicians.

It is possible that the title of this topic is not the name you selected. Please check the Synonyms listing to find the alternate name(s) and Disorder Subdivision(s) covered by this report

This disease entry is based upon medical information available through the date at the end of the topic. Since NORD's resources are limited, it is not possible to keep every entry in the Rare Disease Database completely current and accurate. Please check with the agencies listed in the Resources section for the most current information about this disorder.

For additional information and assistance about rare disorders, please contact the National Organization for Rare Disorders at P.O. Box 1968, Danbury, CT 06813-1968; phone (203) 744-0100; web site or email

Last Updated:  4/9/2012

Copyright  1984, 1985, 1987, 1988, 1990, 1992, 1996, 1997, 1998, 1999, 2000, 2002, 2003, 2008, 2012 National Organization for Rare Disorders, Inc.

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