Most acute myeloid leukaemia patients with intermediate mutant FLT3/ITD levels do not have detectable bi-allelic disease, indicating that heterozygous disease alone is associated with an adverse outcome.

FLT3 internal tandem duplication mutant levels >50%, indicative of bi-allelic disease in some cells, are associated with a particularly poor prognosis in acute myeloid leukaemia; lower levels have an intermediate prognosis relative to wild-type FLT3. To examine whether a small population of homozygous mutant cells is responsible for the worse relapse risk rather than heterozygous disease per se, we determined the genetic composition of 34 intermediate mutant level (25-50%) samples. Only two had evidence of mutant homozygosity; only one had more homozygous than heterozygous mutant cells. Bi-allelic disease in intermediate mutant level cases is uncommon and heterozygous disease is sufficient for adverse outcome.