Several isoforms of the NMDAR1 glutamate receptor subunit are produced by alternative mRNA splicing of three cassette sequences. Using in situ hybridization with exon-specific probes, we have observed differential regional expression of a cassette in the amino terminus of the NMDAR1 subunit, Insertion I, which confers distinct structural and physiologic properties. The differential distribution of expression is most prominent in the basal ganglia, where only the subthalamic nucleus expresses the insertion at high levels.