FLT3 and leukemia: The mutant allele burden in patients with leukemia is one of the most important features modulating the prognostic impact of the mutation.7, 21, 22 Prognostic relevance of FLT3-ITD allele burden might be based on biological grounds: highest FLT3-ITD corresponds to a homozygous state, usually because of a process of uniparental disomy, whereas a low ITD burden might have arisen in a minor subclone, perhaps occurring at a later stage of the leukemogenic process.23