A mutation in domain I of β₂GPI at positions 43 and 40 abrogates binding of anti-β₂GPI antibodies. A hydrophilic ELISA plate was coated with 10 μg/mL recombinant full-length β₂GPI. IgG (20 μg/mL) of 3 patients (A, B, C) was preincubated with Sepharose beads coated with full-length recombinant β₂GPI or 1 of 3 β₂GPI molecules with point mutations (D8A, G40E, or R43G) or only blocked with BSA. Supernatants were added to the plate and IgG binding was detected by an alkalic-phosphatase–labeled goat antihuman IgG antibody. The OD obtained with IgG incubated with BSA-blocked Sepharose beads was set at 100%. Binding of IgG anti-β₂GPI antibodies to β₂GPI coated onto a hydrophilic ELISA plate can be fully absorbed by full-length recombinant β₂GPI and D8A but only partially by R43G and G40E. Error bars represent mean ± SEM of triplicate points.