Hermansky Pudlak syndrome (HPS) is an autosomal recessive inherited disorder characterized by defects in synthesis and/or secretion of three related subcellular organelles: melanosomes, platelet-dense granules, and lysosomes. In the mouse, mutant forms of any of 14 separate genes result in an HPS-like phenotype. The mouse pearl and mocha genes encode subunits of the AP3 adaptor protein complex, confirming that HPS mutations involve proteins regulating intracellular vesicular trafficking. Therefore, expression of several additional proteins involved in vesicular transport was examined by immunoblotting of platelet extracts from HPS mutant and control mice. Platelet levels of SCAMPS (secretory carrier membrane proteins), Rab11, Rab31, NSF (N-ethylmaleimide-sensitive fusion protein), syntaxin 2, syntaxin 4, munc18c, and p115/TAP (p115/transcytosis-associated protein) were not significantly altered in several different HPS mutants. However, gunmetal (gm/gm) platelets contained decreased amounts of SNAP-23. The Snap23 gene was mapped to mouse chromosome 5, demonstrating it cannot encode the gm gene, which maps to chromosome 14. It is likely therefore that the gm gene functions upstream of SNAP-23 in vesicular trafficking.