We examined the genomic structure of the reeler gene in Orleans reeler mouse mutant. Exon skipping of the reeler gene caused a 220 bp deletion in the transcript, resulting in a frame shift of the reeler gene which disrupts the 8th EGF-like motif of the reeler product. Surprisingly, the skipped exon was inserted by the 7104 bp L1 element which carried the full-length stretch of the mouse L1 sequence, consisting of a 212 bp F-type tandem repeat, open reading frame 1 (ORF1), ORF2, the polyadenylation signal and a poly A stretch. The transposed L1 sequence was flanked by 13 bp of the target sequence at both ends. ORF1 and ORF2 of this L1 repeat element are thought to encode a component of the RNP particle and the reverse transcriptase, respectively. Orleans reeler was originally established by spontaneous mutation caused by L1 insertion, and this L1 sequence is considered to be potentially active for transposition in mouse genome.