CBS is a vitamin B6-dependent transsulfuration enzyme needed to synthesize cysteine from methionine, catalyzing the condensation of serine with homocysteine to form cystathionine. A deficiency of CBS causes homocystinuria (MIM 236200), one of the most prevalent inborn errors, characterized by mental retardation, seizures, psychiatric disturbances, skeletal abnormalities and vascular disorders. Patients with CBS deficiency exhibit a major biochemical abnormality, hyperhomocysteinemia (HHcy), a condition associated with highly elevated plasma homocysteine levels. HHcy is recognized as a risk factor for several neurological diseases, such as cognitive impairment, dementia and Alzheimer's disease. Although the link between CBS deficiency and homocystinuria was first described over 40 years ago and mental retardation was the first clinical feature of the disease to be classified, very little is known about the role of CBS in the CNS. Here we show the regional and cellular distribution of CBS in the adult and developing mouse brain. In the adult mouse brain, CBS was expressed ubiquitously, but most intensely in the cerebellar molecular layer and hippocampal dentate gyrus. Immunohistochemical analysis revealed that CBS is preferentially expressed in cerebellar Bergmann glia and in astrocytes throughout the brain. At early developmental stages, CBS was expressed in neuroepithelial cells in the ventricular zone, but its expression changed to radial glial cells and then to astrocytes during the late embryonic and neonatal periods. Moreover, CBS was significantly accumulated in reactive astrocytes in the hippocampus after kainic acid-induced seizures, and cerebellar morphological abnormalities were observed in CBS-deficient mice. These results support the role of CBS in the development and maintenance of the CNS, and suggest that radial glia/astrocyte dysfunction might be involved in the complex neuropathological features associated with abnormal homocysteine metabolism.