GP1BA and Macrothrombocytopenia: Using mice lacking Cdc42 in MKs and platelets (Cdc42fl/fl Pf4-cre, further referred to as Cdc42−/− (ref. 13)), we have previously shown that Cdc42 controls cytoskeletal dynamics downstream of GPIbα in platelets and that its absence in MKs causes marked macrothrombocytopenia by unknown mechanisms13.