Defective aggregation of EphB2LacZ platelets. Platelet aggregation was recorded for 5 minutes by optical aggregometry using washed platelets obtained from control and EphB2LacZ mice following stimulation with collagen (1 µg/mL [A-B] or 5 µg/mL [C-D]), CRP-XL (0.5 µg/mL [E-F] or 1 µg/mL [G-H]), and thrombin (0.1 U/mL [I-J] or 0.2 U/mL [K-L]). Data represent mean ± SD (n = 3). The level of aggregation obtained with control platelets at 5 minutes was taken as 100% (Student t test: *P ≤ .05, **P ≤ .01, and ***P ≤ .001).