Mutant mice in which beta-galactosidase gene (lacZ) was inserted into fyn locus were generated by homologous recombination in embryonic stem cells to examine the Fyn expression in the central nervous system. In adult brain, intensive beta-galactosidase activity was observed in olfactory bulb, cerebellum and hippocampus of the limbic system; the subcellular distribution of the activity was apparent not only in cell body but also in neural processes, and homozygous mutant mice live-born displayed an anatomical abnormality in the neural cell layer of the hippocampal formation. In spinal cord it was specifically expressed in dorsal horn, and in brain stem it was more characteristic in the sensory pathway, suggesting roles of Fyn in the sensory nervous network. In the white matter area, it was intense at postnatal day 10 but not detectable in adult, suggesting Fyn's role in myelinization.