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Aneuploidy is a change in the number of chromosomes that can lead to a chromosomal disorder. The most recognized form of aneuploidy are the trisomy diseases of Down syndrome and Edwards syndrome. Trisomy may not necessarily be present in all cells in an individual. It may be detected in just a specific tissue or within different cells in a tissue. When the presence of chromosomal abnormalities occurs differentially within an individual, it is called chromosomal mosaicism. In general, as we would expect, individuals who are mosaic for a chromosome change tend to have a less severe form of the syndrome present than full trisomy individuals. Critical examples of mosaicism are found in leukemia cases, specifically; chronic lymphocytic leukemia (CLL) which is a trisomy of chromosome 12 and acute myeloid leukemia (AML) prognosis which is a trisomy of chromosome 8. Aneuploidy is common in cancerous cells.