Mosaicism is caused by an error in cell division very early in the development of the unborn baby. Examples of mosaicism include:

• Mosaic Down syndrome
• Mosaic Klinefelter syndrome
• Mosaic Turner syndrome
• Mosaic neurofibromatosis

Symptoms vary and are very difficult to predict. Symptoms may not be as severe if you have both normal and variant cells.

Genetic testing can diagnose mosaicism.

Tests will likely need to be repeated to confirm the results, and to help determine the type and severity of the disorder. Sometimes two different body tissues may need to be tested, such as blood and skin, or skin from two different areas of the body.

Treatment will depend on the type and severity of the disorder. You may need less intense treatment if only some of the cells are variant.

How well you do depends on which organs and tissues are affected (for example, the brain or heart). It is difficult to predict the effects of having two different cell lines in one person.

In general, people with a high number of variant cells have the same outlook as people with the typical form of the disease (those who have all variant cells). The typical form is also called non-mosaic.

People with a low number of variant cells may be only mildly affected. They may not discover that they have mosaicism until they give birth to a child who has the non-mosaic form of the disease. Sometimes a child born with the non-mosaic form will not survive, but a child born with mosaicism will.

Complications depend on how many cells are affected by the genetic change.

A diagnosis of mosaicism may cause confusion and uncertainty. A genetic counselor may help answer any questions about diagnosis and testing.

There is currently no known way to prevent mosaicism.

Chromosomal mosaicism; Gonadal mosaicism

Bacino CA, Lee B. Chromosome disorders. In: Kliegman RM, St. Geme JW, Blum NJ, et al, eds. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia, PA: Elsevier; 2025:chap 99.

Driscoll DA, Simpson JL. Genetic screening and diagnosis. In: Landon MB, Galan HL, Jauniaux ERM, et al, eds. Gabbe's Obstetrics: Normal and Problem Pregnancies. 8th ed. Philadelphia, PA: Elsevier; 2021:chap 10.

Jelin AC, Van den Veyver IB. Preconception and prenatal screening and diagnosis. In: Cohn RD, Scherer SW, Hamosh A, eds. Thompson and Thompson Genetics and Genomics in Medicine. 9th ed. Philadelphia, PA: Elsevier; 2024:chap 18.

Updated by: Anna C. Edens Hurst, MD, MS, Associate Professor in Medical Genetics, The University of Alabama at Birmingham, Birmingham, AL. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.