HOXA9 and leukemia: MLL-r leukemia cells express a subset of genes including HOXA9 and MEIS1 whose expression is normally confined to immature hematopoietic cells such as hematopoietic stem cells (HSCs) (hereafter we refer to as HSC program genes) (Armstrong et al., 2002; Yeoh et al., 2002; Krivtsov et al., 2006; Somervaille and Cleary, 2006) (Figure 3B).