Inhibition of MEK abolishes VWF-mediated platelet aggregation. Platelets were treated with the MEK inhibitor U0126 (10 μM [+] or its vehicle [–] before stimulation with ristocetin (300 μg/mL) and VWF (8 μg/mL) in the absence (–) or presence (+) of previous treatment with 1 mM aspirin. Platelet aggregation was measured as described in “Materials and methods.”