Leukemia in a trisomy 21 mosaic: specific involvement of the trisomic cells.

Acute monoblastic leukemia was diagnosed in a 32-month-old boy with mild signs of Down's syndrome. Chromosome analysis of cultured skin fibroblasts and peripheral blood lymphocytes disclosed a constitutional mosaicism (46,XY/47,XY,+21). At initial diagnosis of acute leukemia, additional chromosomal changes were found in bone marrow blasts that were consistent with the M5 subtype.