Collectively, these data indicate that MLL-AF9 can immortalize CD34+ cells of both neonatal as well as adult origin in vitro, whereas MLL-AF4 can solely immortalize cells of neonatal origin comparable to KMT2Ar patient leukemia, as the portion of MLL-AF4 leukemia in adults is very rare. The gene discussed is KMT2A; the disease is leukemia.