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Mosaic Down syndrome is a rare Down syndrome variant that involves a genetic condition known as mosaicism. Mosaicism occurs when an individual who developed from a fertilized egg has two or more genetic populations of cells with different genotypes. Whereas people with the more common trisomy 21 form of Down syndrome have an extra chromosome in every cell, those with mosaic Down syndrome have a mixture of cells. In many cases, individuals with mosaic Down syndrome have some cells with the typical forty-six chromosomes and some with an extra copy of chromosome twenty-one. Some cells may even have a third copy of chromosome twenty-one. Mosaic Down syndrome typically accounts for only about 2 percent of all Down syndrome cases. While the symptoms of mosaic Down syndrome are similar to those associated with trisomy 21 Down syndrome, the presence of at least some normal cells means that the presentation of these symptoms is usually less severe.