Cystathionine beta-synthase (CBS; EC 4.2.1.22) is a key enzyme in the generation of cysteine from methionine. A deficiency of CBS leads to homocystinuria, an inherited human disease characterized by mental retardation, seizures, psychiatric disturbances, skeletal abnormalities, and vascular disorders; however, the underlying mechanisms remain largely unknown. 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 it is expressed 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. CBS was most highly expressed in juvenile brain, and a striking induction was observed in cultured astrocytes in response to EGF, TGF-alpha, cAMP, and dexamethasone. 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. Taken together, these results suggest that CBS plays a crucial role in the development and maintenance of the CNS and that radial glia/astrocyte dysfunction might be involved in the complex neuropathological features associated with abnormal homocysteine metabolism.