Thrombotic thrombocytopenia following COVID-19 adenoviral vector vaccines mimics the clinical image of autoimmune HIT by showing strong reactivity on a PF4-heparin enzyme-linked immunosorbent assay (ELISA), due to high-titer immunoglobulin G class antibodies directed against PF4 with enhanced platelet activation in the presence of PF4, but also inhibition, rather than activation, when adding low molecular weight heparin (LMWH) [2,3,7]. The gene discussed is PF4; the disease is COVID-19.