Modulator-induced VWF binding. Cells expressing either wild-type or mutant GP Ibα were incubated with increasing concentrations of VWF in the presence of (A) 2.0 μg/mL botrocetin or (B) 1.5 mg/mL ristocetin. Cells were then incubated with HRP-conjugated anti–human VWF antibody. Results were normalized based on GP Ibα expression, as determined by the binding of WM23. Error bars indicate SEM. ^*Significant differences (P < .05).