Further support is provided by twins discordant for childhood ALL, in which preleukemic clones harboring the initiating lesion, such as ETV6-RUNX1 fusion, have been found to persist for several years in the healthy co-twin without leading to the development of leukemia, supporting the supposition that additional mutations are required for overt leukemia development [19,24,26,27]. The gene discussed is RUNX1; the disease is acute lymphoblastic leukemia.