Chromosome number

Normal cells have 46 chromosomes (22 pairs and the sex chromosomes—XX female and XY male). Some leukemia cells contain extra copies of entire chromosomes and thus can contain more than 46 chromosomes. This is called hyperdiploidy, and it is found in approximately 27 percent of children with ALL. There are two ways to evaluate the number of chromosomes: counting them (karyotyping) or measuring the DNA content of cells (DNA index). Children with 53 or more chromosomes per cell or a DNA index greater than 1.16. have a very good prognosis. Hyperdiploidy occurs most often with B-lineage ALL.

Extra copies of particular chromosomes also seem to indicate a very favorable prognosis. Children with extra copies of chromosome 4, 10, and 17 have an especially favorable prognosis.

Approximately 6 percent of children with ALL have fewer than 45 chromosomes in the leukemia cells. This is called hypodiploidy and carries a poor prognosis.

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