Null mutations in the retinal degeneration B gene (rdgB) in flies result in an activity-dependent retinal degeneration. Here we report the isolation of the mouse and human homologues of rdgB gene that are strongly expressed in brain and moderately expressed in other tissues. The deduced amino acid sequences encoding a 1244 a.a protein bear a 96% similarity between mouse and human and resemble the Drosophila rdgB, particularly in the phosphatidylinositol transfer domain at the N-terminus and in six putative transmembrane domains at the C-terminus. Immunoblots with antiserum raised against a bacterially expressed fragment of the mouse rdgB showed the band with a molecular weight of about 170 kDa. Interestingly, a burst of mouse rdgB expression occurs on 17th day of gestation, suggesting a crucial role of the gene product in brain development at this particular stage. A gene, mpt-1, encoding for mouse rdgB was mapped to the proximal end of chromosome 19, which is the same location as Mvb-1, a gene locus encoding the modifier of mouse vibrator mutation (mv).