Using a pre-leukemia mouse model, in which expression of Mll-AF4 initiates in all definitive hematopoietic cells formed during embryonic development (Mll-AF4 invertor mouse crossed with VEC-Cre), we previously identified the fetal liver as the starting point of MLL-AF4-driven leukemogenesis 4, 7. The gene discussed is AFF1; the disease is leukemia.