Binding of quinine-induced thrombocytopenia patient sera to WT and chimeric GPIXs. Sera from 6 patients were used against the CHOαβ cells expressing WT and chimeric GPIXs to determine what region of GPIX was the target of antibodies in patient sera. Histograms represent the reaction of one patient serum (patient 1) against the cell lines expressing WT GPIX and the 4 chimeras. The pattern of reaction was representative of the set of 6 sera tested. Solid peaks (gray) denote the binding of negative controls [(–) quinine test samples and isotype-matched IgG]. Open peaks (black) denote patient-derived quinine-dependent antibody binding.