Pallid (pa) is 1 of 13 platelet storage pool deficiency (SPD) mouse mutants. pa animals suffer from prolonged bleeding time, pigment dilution, kidney lysosomal enzyme elevation, serum alpha1-antitrypsin activity deficiency and abnormal otolith formation. As with other mouse mutants of this class, characterization of pa mice suggests a defect in organelle biosynthesis. Here we describe the physical mapping, positional cloning, and mutational and functional analysis of the gene that is defective in pa mice. It encodes a ubiquitously expressed, highly charged 172-amino-acid protein (termed pallidin) with no homology to known proteins. We detected a nonsense mutation at codon 69 of this gene in the pallid mutant. In a yeast two-hybrid screen, we discovered that pallidin interacts with syntaxin 13, a t-SNARE protein that mediates vesicle-docking and fusion. We confirmed this interaction by co-immunoprecipitation assay. Immunofluorescence studies corroborate that the cellular distribution of pallidin overlaps that of syntaxin 13. Whereas the mocha and pearl SPD mutants have defects in Ap-3, our findings suggest that pa SPD mutants are defective in a more downstream event of vesicle-trafficking: namely, vesicle-docking and fusion.