Cyfip1-deficient platelets cannot form branched actin filaments. (A) Platinum replica electron microscopic images of the cytoskeleton ultrastructure of platelets spread on fibrinogen after 15 minutes. Scale bars, 1 or 4 µm. (B) Left, F-actin content under resting conditions and upon stimulation with CRP (10 μg/ml) or thrombin (0.1 U/ml) was assessed by measuring the MFI of bound FITC-labeled phalloidin in flow cytometry. Right, Ratio of MFI of activated vs resting samples reflecting F-actin polymerization rates (n = 4, representative for 3 independent experiments). Values are mean plus or minus SD. *P < .05.