Syk and SLP-76 are required for aggregation, TxA2 production, and ATP secretion; Lyn is required for the phosphorylation of Syk and SLP-76; and Syk is required for the phosphorylation of SLP-76 in response to bt/VWF. (A) Syk-deficient and SLP-76–deficient platelets each agglutinated, but did not aggregate in response to bt/VWF stimulation, and (B) did not produce TxA2 or secrete ATP. (C) Immunoprecipitation (IP) was run on platelet lysate as described in “Immunoprecipitation and Western blotting” using monoclonal antibody 4G10. Syk was not phosphorylated in Lyn-deficient platelets in response to bt/VWF. (D) SLP-76 was not phosphorylated in Syk-deficient platelets in response to bt/VWF. The error bars represent standard deviation; n = 3. In some cases, the values of the SD were so small that the bars cannot be seen.