Role of Rho GTPases in HSC/P homing, engraftment, and mobilization. Rac1, Rac2, Cdc42, and Rho control different hematopoietic stem and progenitor cell (HSC/P) functions. Signals required for HSC self-renewal are mediated by Rho, Cdc42, and Rac1. Although Rac1 appears to be required for proliferation, Rac2 controls survival. Cdc42 is necessary for cell-cycle entry of quiescent (G₀) HSC/Ps in the bone marrow (BM) microenvironment, proliferation, and aging of HSC/Ps. Rac1 and Cdc42 control homing and interaction with the BM niche, whereas combined Rac1 and Rac2 activities and Cdc42 are necessary for the retention of HSC/Ps in the BM.