PF4 and Thrombocytopenia: KKO (Anti–human PF4 monoclonal antibodies to PF4; KKO causes thrombocytopenia in an in vivo model of HIT) and RTO (Anti–human PF4 monoclonal antibodies to PF4, RTO does not cause thrombocytopenia in an in vivo model of HIT) have been characterized, including their amino acid sequences [83].