The genomic sequence extending from the 3' end of intron 1 through the 5' end of exon 3 was replaced with the missing portion of intron 1 through the 5' end of exon 3 following by a puromycin resistance cassette, loxP site, 3'Hprt mini-gene, and a duplication of the 3' end of intron 1 through the 5' end of exon 3. The absence of transcript expression was confirmed by northern blot analysis on brain extracts.