MLL fusions regulate the activity of Rac GTPases. (A-B) Western blot analysis of active Rac pull-down assays. Rac1 GTP and Rac2 GTP levels, and total Rac protein expression are shown in (A) preleukemic and leukemic conditional MLL-ENL cells (data are representative of 3 independent experiments) and (B) preleukemic conditional MLL-AF9 cells (data are representative of 2 independent experiments). Numbers represent densitometric quantitation of Rac1 GTP levels normalized to total Rac1 protein bands. α-tubulin served as a loading control. (C) Model for generation of preleukemic and leukemic MLL fusion cells. (D) Rac1 GTP and Rac2 GTP levels, and total Rac protein expression, in leukemic constitutive MLL-ENL cells, treated with and without doxycycline for 72 hours. α-tubulin served as a loading control. Data were obtained from individual preleukemic and leukemic cell lines in each case and are representative of 2 independent experiments.