The gene encoding the synaptosomal-associated protein--25 kDa (SNAP-25) was mapped by analysis of somatic cell hybrids and an intersubspecies backcross to mouse Chromosome 2. To identify potential mutants for SNAP- 25, mice bearing mutations mapping to this region of Chromosome 2 were screened for Snap gene abnormalities. Mice heterozygous for the semidominant mutation coloboma (Cm/+) were identified that carried a deletion of Snap gene sequence. Analysis of genomic DNA revealed that the Snap gene dosage in Cm/+ mice was 50% lower than control littermates. Additionally, SNAP-25 mRNA and protein expression were 50% lower in coloboma mice than control littermates. The coloboma mouse phenotype is characterized by small eyes and head bobbing; in addition, we observed that these mice were extremely hyperactive with spontaneous locomotor activity exceeding three times control mouse activity. The localization of the genetic abnormality in coloboma mice using the Snap gene marker will provide a powerful tool for studying the biologic basis of locomotor hyperactivity.