Altered localization of MRP4 in platelets from HPS patients. Platelets adhered to collagen were stained by the anti-MRP4 antibody SNG and a tetramethylrhodamine isothiocyanate–labeled secondary antibody (1:200) as described in “Patients, materials, and methods.” Differences in fluorescence were enhanced using pseudocolor (× 100 magnification, MetaMorph Imaging series 4.6 software, Visitron Systems, Puchheim, Germany). Control platelets (A) show membrane staining and 1 to 3 strong staining clusters, most likely resembling dense granules, whereas HPS platelets mostly showed membrane staining only (B). Higher magnification of HPS platelet (C).