Leukemia in a black child with Bloom's syndrome: somatic recombination as a possible mechanism for neoplasia.

A 5 1/2-year-old child with Bloom's syndrome developed acute lymphocytic leukemia (ALL). Bloom's syndrome is associated with chromosomal aberrations, and affected individuals have an increased incidence of leukemia and solid tumors. The skin on our patient had adjacent areas of decreased and increased pigmentation similar to the "twin-spots" seen in Drosophila. "Twin-spots" are the manifestation of somatic cell DNA recombination and provide evidence that clones of cells in Bloom's syndrome have become homozygous for a particular gene. Somatic cell recombination is proposed as a mechanism to explain the increased incidence of neoplasia in Bloom's syndrome and supports the hypothesis that cancer may be a recessive disorder at the cellular level.