In seminal studies conducted in the early 2000s using retroviral-mediated gene transfer, the groups of J. Gary Gilliland, Guy Sauvageau, and Keith Humphries provided the first evidence of the transforming activity of leukemia-associated aberrant genes, including those encoding constitutively active variants of regulatory molecules, such as mutants of the tyrosine kinase receptor FLT3 bearing internal tandem duplications in the juxta-membrane region (FLT3-ITD) or the fusion proteins NUP98-HOXA9 or NUP98/HOXD13. The gene discussed is FLT3; the disease is leukemia.