Heparin-induced thrombocytopenia (HIT) is a prothrombotic immune reaction to unfractionated and low-molecular-weight heparin (LMWH) products mediated by platelet-activating antibodies against complexes of platelet factor 4 (PF4) and heparin.1, 2Though venous thromboembolism is most common in medical patients, arterial thromboembolism can occur and is associated with an increased risk of mortality and morbidity, including limb ischemia, amputation, and stroke.1HIT is suspected when the baseline platelet count decreases by 50%. The gene discussed is PF4; the disease is limb ischemia.