Binding of polyclonal IgG from patients with antiphospholipid syndrome (APS) to Escherichia coli–expressed recombinant human domain I in the solid phase. APS, n = 22; systemic lupus erythematosus (SLE) controls, n = 20; healthy controls, n = 10. For APS patients versus SLE/autoimmune controls, P = 0.0004; for APS patients versus healthy controls, P < 0.0001; for SLE/autoimmune controls versus healthy controls, P = 0.39. Bars show the mean.