KMT2A and acute lymphoblastic leukemia: Unlike other pediatric ALLs (with a 5-year survival of ~ 90%), MLL1-rearranged (MLL1-r) ALL shows a poor prognosis with 5-year survival rates of 34–39% [1–4], while MLL1-r AML patients have similarly poor outcomes to other AMLs with 5-year survival rates of ~ 50% for younger (< 45 years) and < 35% for older patients [5].