Rac1/Cdc42^−/− platelets display abnormal ultrastructure and are rapidly cleared from the circulation. (A-B) Representative TEM pictures are shown. (A) Normal ultrastructure of wt, Rac1^−/−, and Cdc42^−/− platelets. Bar, 0.5 µm. (B) Abnormal ultrastructure of Rac1/Cdc42^−/− platelets. Bar sizes are depicted. (C) Increased number of marginal microtubular rings in Rac1/Cdc42^−/− platelets. Bar sizes are depicted. (D) Life span from Rac1^−/− (dark gray), Cdc42^−/− (light gray), and Rac1/Cdc42^−/− (patterned) platelets compared with wt (black) platelets. (E) Infinite tail bleeding times in Rac1/Cdc42^−/− (patterned) compared with wt (black) mice (n = 7 per group; ***P < .001).