Some studies showed that severe FXI deficiency with residual FXI: C (FXI coagulant activity) < 15% could prevent stroke and venous thromboembolism (Salomon et al., 2008; Salomon et al., 2011), while others reported that even moderate FXI deficiency (residual FXI activity of 30%–50%) could prevent venous thrombosis and cardiovascular events (Preis et al., 2017). The gene discussed is F11; the disease is venous thromboembolism.