FAK-null platelets support defective hemostasis and exhibit defective spreading on immobilized fibrinogen. (A) Tail rebleeding assays were performed on 10 mice in each group; the data are expressed as mean (horizontal bar), SEM (box), and data spread (vertical bar). (B) Platelet spreading was quantified by image analysis software and expressed as surface coverage; the numbers represent mean surface coverage (± SEM). **P < .01; ***P < .001. The data are representative of 3 independent experiments. (C) Platelets were allowed to spread on immobilized fibrinogen for 1 hour in the presence or absence of agonists before vinculin (red) and β3 integrin (green) staining. Scale bar equals 4 μm; white boxes indicate magnified areas.