Modulator-induced VWF binding to alanine mutants. Cells expressing either wild-type GP Ibα or alanine mutants were incubated with increased 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 surface expression, as determined by the binding of the antibody WM23. Error bars indicate SEM.