Granule release is defective in syntaxin-11–deficient human platelets. Platelet extracts (5.0 × 10⁷ platelets/lane) from control patients and an FHL4 patient were probed by Western blotting with the indicated Abs (A). [³H]-Serotonin labeled platelets from the FHL4 patient (gray circle symbols) and a normal control donor (black square symbols) were prepared as in “Methods” and stimulated with either the indicated thrombin concentration at room temperature for 1 minute (B) or 0.05 U/mL of thrombin for the indicated time points (C). Release of [³H]-serotonin from dense granules, PF4 from α-granules, and β-hexosaminidase from lysosomes was measured and calculated as described in the legend to Figure 1.