Scheme of Rho GTPase involvements in lymphocyte development. Rac1 and Rac2 are important for common lymphoid progenitor (CLP) differentiation from hematopoietic stem cells (HSCs) in the BM. Rac1 and Rac2, as well as RhoH, regulate T-cell development in the thymus by affecting β-selection and positive selection. Rac2 is also required for Th1 differentiation in peripheral lymphoid tissues. Rac1, Rac2, and Cdc42 are critical for multiple stages of B-cell development in the spleen, whereas Rac2, Cdc42, and RhoG also regulate antibody production. DN indicates CD4⁻CD8⁻ double-negative thymocytes; DP, CD4⁺CD8⁺ double-positive thymocytes; SP, CD4⁺ or CD8⁺ single-positive T cells; Th, helper T cells; Tc, cytotoxic T cells; NFB, newly formed B cells; MZB, marginal zone B cells; and FOB, follicular B cells.