In the mouse, more than 16 loci are associated with mutant phenotypes that include defective pigmentation, aberrant targeting of lysosomal enzymes, prolonged bleeding, and immunodeficiency, the result of defective biogenesis of cytoplasmic organelles: melanosomes, lysosomes, and various storage granules. Many of these mouse mutants are homologous to the human Hermansky–Pudlak syndrome (HPS), Chediak–Higashi syndrome, and Griscelli syndrome. We have mapped and positionally cloned one of these mouse loci, buff (bf), which has a mutant phenotype similar to that of human HPS. Mousebfresults from a mutation inVps33aand thus is homologous to the yeast vacuolar protein-sorting mutantvps33andDrosophilacarnation (car). This is the first found defect of the class C vacuole/prevacuole-associated target solubleN-ethylmaleimide-sensitive factor attachment protein receptor (t-SNARE) complex in mammals and the first mammalian mutant found that is directly homologous to avpsmutation of yeast.VPS33Athus is a good candidate gene for a previously uncharacterized form of human HPS.