Representation of APS autoantibody binding. (A) aCL assay. Shown is the autoantibody (Y) binding negatively charged (−) phospholipid. (B) β2GPI-dependent aCL assay. Shown is the autoantibody binding to domain I of β2GPI. β2GPI domains are denoted by the numbers 1-5. (C) Anti-β2GPI domain I assay.