It is postulated that the ETV6-RUNX1 fusion gene, which is the molecular consequence of t (12;21) (p13;q22) and is present in approximately 25% of cases of childhood B-cell precursor ALL, is acquired in utero, but it requires additional post-natal somatic mutations in order to result in the development of overt leukemia. The gene discussed is RUNX1; the disease is leukemia.