Binding of mAbs measured by flow cytometry to resting platelets and CHO cells transfected with wild-type or mutant α_IIbβ₃. (A) Histograms showing increased PAC-1 binding to resting platelets from patients II.4 and III.5 versus control, but no difference of fibrinogen binding or P-selectin surface expression. (B) Histograms showing similar expression levels of α_IIbβ₃ (with antibody PL2-73) in all CHO cell lines and increased PAC-1 binding to the α_IIbβ₃-H723 and α_IIbβ₃-A723 (reversed in the presence of 1 mM RGDS). Irrelevant mouse IgG was used to determine nonspecific antibody binding.