Determination of the activation state of the mutated _IIbβ₃ complex in a stable CHO cell line using flow cytometry. Cells stably transfected with wild-type β₃ and wild-type _IIb or wild-type β₃ and mutated _IIb were incubated with FITC–PAC-1 in the presence or absence of 2 μmol/L anti-LIBS6 as detailed in Materials and Methods. Log of the fluorescence is on the abscissa and the number of cells examined is on the ordinate. A total of 5,000 cells were analyzed. PAC-1 binding was assessed in the presence (░) or absence (□) of 1 mmol/L RGDS peptide as shown. Note that PAC-1 binding requires the presence of the activating MoAb and was inhibited by RGDS.