Deletion of syntaxin-2 (Stx2) or syntaxin-4 (Stx4) has no effect on platelet secretion. Platelet extracts (5.0 × 10⁷ platelets/lane) were prepared from wild type (Wt) and syntaxin-2–knockout (Stx2 KO; A) or syntaxin-4–knockout (Stx4 KO; C) mice and the indicated proteins were detected by Western blotting. [³H]-serotonin–labeled platelets from Stx2 KO (B) or Stx4 KO (D; gray circle symbols) or Wt (black square symbols) were prepared as described in “Methods” and were stimulated with the indicated thrombin concentrations for 1 minute. Release of [³H]-serotonin from dense granules, PF4 from α-granules, and β-hexosaminidase from lysosomes was measured and the percentage of secretion was calculated. The data are the averages of triplicate measurements with the SD indicated.