Munc18b is the major Munc18 isoform in platelets and is missing in platelets from FHL5/Munc18b/STXBP2patients. Recombinant Munc18a, b, and c were used to generate a standard curve for quantification (using ECF Western blotting) of each Munc18 isoforms in the indicated number of human platelets (A). Equal amounts of each Munc18 isoform were subjected to Western blotting with the antibodies used in this study to evaluate their cross-reactivity (B). Platelet extracts (1.0-2.0 × 10⁷ platelets/lane) from control and FHL5 patients were probed by Western blotting with the indicated antibodies (C), and the ratio of patient to control levels of each protein was calculated and graphed (D).