Expression of Hox genes (e.g. Hoxa9) is upregulated in human leukaemias carrying MLL rearrangements, and it has been shown recently that transformation by MLL fusion proteins requires Hoxa7 and Hoxa9 (Ayton and Cleary, 2003), indicating that these are pivotal for MLL-associated leukaemogenesis. The gene discussed is KMT2A; the disease is leukemia.