Acute myelogenous leukaemia with t(8;21) translocation of normal cell origin in mosaic Down's syndrome with isochromosome 21q.

We report a 13-year-old girl with Down's syndrome (DS) having a mosaic karyotype of 46,XX/46,XX, -21,+i(21q), who developed acute myelogenous leukaemia (AML) (FAB M1). The t(8;21) translocation generating a AML1/MTG8 chimaeric gene of her blasts was demonstrated by cytogenetic analysis and reverse transcription-polymerase chain reaction. Interestingly, the leukaemic clone with t(8;21) did not have isochromosome 21q, indicating that the blasts were of normal cell origin. These findings suggest that, in older patients with DS, 21 trisomy cells have no greater predisposition to develop AML than normal karyotypic cells.