A summary of the known involvement of Rho GTPases in the regulation of hematopoiesis based on mouse gene–targeting studies. Cdc42, Rac1, Rac2, RhoH, RhoG, and RhoA are among the best understood Rho GTPase family members for which mouse gene–targeting models have been characterized in various blood lineages. LTR indicates long-term repopulating; and STR, short-term repopulating. Other abbreviations of various progenitors are described in the text.