Corticosteroid binding globulin (CBG, transcortin) has been shown to be expressed in the brain of rat and human species. In this study, we examined the CBG brain expression and cDNA structure in mice, comparing wild-type (Cbg(+/+)) and Cbg knockout mice (Cbg(-/-), obtained by genetic disruption of the SerpinA6 alias Cbg gene). We used double immunofluorescence labeling with specific neuronal and glial markers to analyze the cellular localization of CBG in various regions of the mouse brain. In wild-type (Cbg(+/+)) mice, we found CBG immunoreactivity in neuronal perikarya of the magnocellular hypothalamic nuclei, amygdala, hippocampus, cerebral cortex, cerebellum and pituitary. A portion of glial cells (astrocytes, oligodendrocytes) contained CBG immunoreactivity, including some of the ependymal cells and choroid plexus cells. No CBG immunoreactivity was detected in Cbg(-/-) brain tissues. Using RT-PCR, we showed that the full-length Cbg mRNA is present in those regions, indicating an intrinsic expression of the steroid-binding globulin. Furthermore, sequencing analysis showed that Cbg cDNA obtained from the mouse hypothalamus was homologous to Cbg cDNA obtained from the liver. Finally, we have evaluated the relative levels of CBG expression in various brain regions and in the liver by quantitative PCR. We found that brain levels of Cbg mRNA are low compared with the liver but significantly higher than in CBG-deficient mice. Although derived from the same gene as liver CBG, brain CBG protein may play a specific or complementary role that requires the production and analysis of brain-specific Cbg knockout models.