A parameter summarizing APC inhibition of thrombin generation with increasing APC concentrations (IC(50)-APC) was increased in all patient groups compared to controls: median values were 15.3 (interquartile range, IQR, 9.7–34.0) in patients with primary APS, 27.3 (IQR 23.5–43.5) in patients with SLE without APS, 64.1 (IQR 25.9–65.0) in patients with SLE/APS compared to 10.4 [IQR 8.5–15.8] in controls, respectively p = 0.003, p = 0.0001 and p = 0.0001. The gene discussed is APC; the disease is systemic lupus erythematosus.