A biochemical model of Rac GTPase involvement in hematopoietic cell transformation. Rac1 has unique as well as redundant roles with Rac2 and Rac3 as signal transducers in multiple oncogene or tumor suppressor gene–mediated leukemogenesis or lymphomagenesis, in addition to their physiologic roles in mediating receptor-stimulated signaling. They are required for p210 BCR-ABL–induced HSC transformation, and MLL-AF9–induced HSC/GMP transformation. Targeting of Rac GTPases has been suggested as a potential therapeutic means in the blood malignancies.