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. Here, PF4 is linked to Macrothrombocytopenia.