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IN SITU hybridization analyses have revealed drastic changes in expression and distribution of five subunit mRNAs of the mouse NMDA receptor channel during brain development. The ω1 subunit mRNA is expressed postnatally and widely in the brain. On the other hand, the ω2 subunit mRNA is found throughout the entire embryonic brain, but its expression becomes restricted to the forebrain at postnatal stages. The ω3 subunit mRNA appears postnatally and predominantly in the cerebellum, whereas the ω4 subunit mRNA is abundantly expressed in the diencephalon and the brainstem at embryonic and neonatal stages. In contrast, the ζ1 subunit mRNA distributes ubiquitously in the brain throughout development. These findings suggest that changes in the subunit composition of the NMDA receptor channel take place during brain development.