International Journal of Cell Biology the stoichiometry between PU.1 and Gata-1. Zhang et al. showed by EMSA (electrophoretic mobility shift assays) that PU.1 directly interacts with the Gata-1 DNA recognition motif thereby blocking its DNA-binding activity [33, 34]. Gata-1 transactivation which ordinarily promotes normal erythroid diﬀerentiation is consequently inhibited and allows erythroleukaemic transformation [33, 44]. Studies have also revealed that deletion of a −14 kb upstream regulatory region (URE) reduces PU.1 expression by 80% in mice and results in acute myeloid leukaemia (AML) and T-cell lymphoma [10, 13]. These mice have an accumulation of immature myeloblasts and neutrophils in both the bone marrow and spleen accompanied by an increased number of immature white blood cells in peripheral blood. Persistent PU.1 expression also induces transformation of early T- cell progenitors, which subsequently develop into aggressive lymphomas [10]. Lastly, a series of PU.1 mutations have been shown to associate with AML in humans [43]. These muta- tions are thought to abrogate PU.1 function by a number of mechanisms, for example, by reducing its DNA-binding capacity or by disrupting its interaction with coregulators or other transcription factors. However, attempts by a number of other groups have failed to demonstrate an association between PU.1 mutations and AML [45–47], suggesting that the mutations identiﬁed in the Mueller study may be linked to rare subsets of the disease. Nonetheless, this does not ex- clude the possibility that PU.1 haploinsuﬃciency can pro- mote leukaemogenesis in humans, further emphasising the importance of maintaining precise PU.1 expression levels in haematopoietic cells. 7. Conclusion Through the study of a variety of transgenic mouse models and cell lines, PU.1 has emerged as a key regulator of haematopoiesis. The expression of PU.1 is exquisitely and dynamically controlled throughout the various haematopoi- etic diﬀerentiation pathways. Precise PU.1 levels direct cell fate by regulating gene expression in a graded manner and by antagonising the function of other haematopoietic re- gulators, such as Gata-1. Disturbing the normal expression pattern of PU.1 in haematopoietic cells can lead to skewed lineage commitment and in some instances, oncogenesis. Indeed, PU.1 haploinsuﬃciency has been linked with AML in humans. Understanding the mechanisms by which PU.1 and other factors regulate haematopoietic diﬀerentiation is of signiﬁcant importance, as manipulation of these factors may oﬀer a therapeutic option for the treatment of leukaemias and other haematopoietic disorders. References [1] F. Moreau-Gachelin, A. Tavitian, and P. Tambourin, “Spi- 1 is a putative oncogene in virally induced murine ery- throleukaemias,” Nature, vol. 331, no. 6153, pp. 277–280, 1988. [2] R. P. DeKoter and H. Singh, “Regulation of B lymphocyte and macrophage development by graded expression of PU.1,” Science, vol. 288, no. 5470, pp. 1439–1441, 2000. 5 [3] P. Kastner and S. Chan, “PU.1: a crucial and versatile player in hematopoiesis and leukemia,” International Journal of Bio- chemistry and Cell Biology, vol. 40, no. 1, pp. 22–27, 2008. [4] E. W. Scott, M. C. Simon, J. Anastasi, and H. Singh, “Require- ment of transcription factor PU.1 in the development of multiple hematopoietic lineages,” Science, vol. 265, no. 5178, pp. 1573–1577, 1994. [5] S. R. McKercher, B. E. Torbett, K. L. Anderson et al., “Targeted disruption of the PU.1 gene results in multiple hematopoietic abnormalities,” EMBO Journal, vol. 15, no. 20, pp. 5647–5658, 1996. [6] J. Back, A. Dierich, C. Bronn, P. Kastner, and S. Chan, “PU.1 determines the self-renewal capacity of erythroid progenitor cells,” Blood, vol. 103, no. 10, pp. 3615–3623, 2004. [7] J. Back, D. Allman, S. Chan, and P. Kastner, “Visualizing PU.1 activity during hematopoiesis,” Experimental Hematology, vol. 33, no. 4, pp. 395–402, 2005. [8] S. L. Nutt, D. Metcalf, A. D’Amico, M. Polli, and L. Wu, “Dynamic regulation of PU.1 expression in multipotent hema- topoietic progenitors,” Journal of Experimental Medicine, vol. 201, no. 2, pp. 221–231, 2005. [9] M. J. Klemsz, S. R. McKercher, A. Celada, C. van Beveren, and R. A. Maki, “The macrophage and B cell-speciﬁc transcription factor PU.1 is related to the ets oncogene,” Cell, vol. 61, no. 1, pp. 113–124, 1990. [10] F. Rosenbauer, B. M. Owens, L. Yu et al., “Lymphoid cell growth and transformation are suppressed by a key regulatory element of the gene encoding PU.1,” Nature Genetics, vol. 38, no. 1, pp. 27–37, 2006. [11] F. Moreau-Gachelin, F. Wendling, T. Molina et al., “Spi-1/PU.1 transgenic mice develop multistep erythroleukemias,” Mole- cular and Cellular Biology, vol. 16, no. 5, pp. 2453–2463, 1996. [12] H. Iwasaki, C. Somoza, H. Shigematsu et al., “Distinctive and indispensable roles of PU.1 in maintenance of hematopoietic stem cells and their diﬀerentiation,” Blood, vol. 106, no. 5, pp. 1590–1600, 2005. [13] F. Rosenbauer, K. Wagner, J. L. Kutok et al., “Acute myeloid leukemia induced by graded reduction of a lineage-speciﬁc transcription factor, PU.1,” Nature Genetics, vol. 36, no. 6, pp. 624–630, 2004. [14] A. Dakic, D. Metcalf, L. Di Rago, S. Mifsud, L. Wu, and S. L. Nutt, “PU.1 regulates the commitment of adult hematopoietic progenitors and restricts granulopoiesis,” Journal of Experi- mental Medicine, vol. 201, no. 9, pp. 1487–1502, 2005. [15] R. C. Fisher, J. D. Lovelock, and E. W. Scott, “A critical role for PU.1 in homing and long-term engraftment by hematopoietic stem cells in the bone marrow,” Blood, vol. 94, no. 4, pp. 1283– 1290, 1999. [16] K. Akashi, D. Traver, and L. I. Zon, “The complex cartography of stem cell commitment,” Cell, vol. 121, no. 2, pp. 160–162, 2005. [17] J. Adolfsson, R. Ma˚nsson, N. Buza-Vidas et al., “Identiﬁcation of Flt3+ lympho-myeloid stem cells lacking erythro-mega- karyocytic potential a revised road map for adult blood lineage commitment.,” Cell, vol. 121, no. 2, pp. 295–306, 2005. [18] A. D. Friedman, “Transcriptional control of granulocyte and monocyte development,” Oncogene, vol. 26, no. 47, pp. 6816– 6828, 2007. [19] J. Seita and I. L. Weissman, “Hematopoietic stem cell: self-re- newal versus diﬀerentiation,” Wiley Interdisciplinary Reviews: Systems Biology and Medicine, vol. 2, no. 6, pp. 640–653, 2010.