Decreased Syk phosphorylation in Sema4D^(−/−) platelets. Platelets from matched Sema4D^(+/+) and Sema4D^(−/−) mice were stimulated with either (A) 5 μg/mL or (B) 10 μg/mL collagen. Lysates were prepared and immunoprecipitated with anti-Syk followed by immunoblotting with the anti-phosphotyrosine antibody, 4G10P (mean ± SEM, N = 3). N.S. = not significant.