Two alternate spliced transcripts of N-methyl-D-aspartate receptor (NR2B) gene derived from a common promoter have been reported earlier. In the present study, we have examined the expression of alternative splice variants of NR2B gene using 5' rapid amplification of C terminal end (RACE) in the RNA derived from mouse cortex. RT-PCR was performed to confirm the RACE data. Here we report the occurrence of three transcripts containing a novel alternate spliced exon, we refer to as exon 1'. This new exon is located upstream of the transcription start site identified by Klein et al. [Gene 208 (1998) 259] in the 5' untranslated region (UTR) of NR2B, thus making a new alternate 5' end of the NR2B transcript. The molecular nature of the transcripts containing the new exon appears to be highly heterogeneous. These results are interesting in that they are of potential importance in translational regulation of the NR2B gene.