Deletions of chromosome 21 restricted to the leukemic cells of children with Down syndrome and leukemia.

Down syndrome (DS) is associated with an increased risk of developing hematological malignancies, but the basis for this predisposition is so far unknown. Using fluorescence in situ hybridization with a panel of locus-specific probes on normal and leukemic metaphases, we have found long-arm interstitial deletions of one of the chromosome 21s in the leukemic cells from five patients with DS and leukemia. This finding provides strong evidence for a gene or genes present on chromosome 21 having an important function in the development of leukemia in individuals with Down syndrome.