MEIS1 and leukemia: Although PIM1, CLDN1 (S4A Fig, Mouse #2–5), and PIM3 (S4B Fig, Mouse #2–7) proto-oncogenes were identified in our screen, their role in leukemia induction is not clear given the well-known co-operative nature of MEIS1 and anterior Hox genes such as HoxA7, 8, 9[17–20].