A targeting vector was designed to delete the bHLHO domain, which is encoded by exons 2 and 3. A tau-lacZ fusion cassette was inserted in frame with the 12th codon in exon 2, followed by a floxed neomycin resistance cassette. The neo was subsequently removed by cre-mediated recombination. Absence of transcript was confirmed by RT-PCR. Expression of the mutant protein was confirmed by in situ hybridization. (J:119562)