Platelets from FHL5/Munc18b/STXBP2 patients have a secretion defect. Platelet-rich plasma was generated by centrifugation and labeled with [³H]-serotonin. After washing, platelets were stimulated with increasing concentrations of thrombin for 1 minute, and then the release of [³H]-serotonin (Dense), PF4 (Alpha), and β-hexosaminidase (Lysosomes) were measured as described under “Platelet secretion analysis” (A and B). (A) Release profiles for biallelic patient 1 (open symbols) and a normal control (closed symbols). Patient 2 showed a similar profile. (B) Release profiles for heterozygous patient 3 (open symbols) and a normal control (closed symbols). [³H]-serotonin, PF4, and β-hexosaminidase in the releasate and remaining in the platelets were measured, and a percent secretion was calculated for each treatment condition. The data are the averages of triplicate measurements with the SD indicated.