The start codon for the B1b isoform of Gabbr1 was converted into a stop codon to prevent translation of the B1b isoform. A floxed neomycin cassette was inserted in the introns between exons 5a and 1b. The neomycin cassette was removed by crossing with mice expressing Cre-recombinase under the control of the cytomegalus virus promoter. Northern blot analysis confirmed absence of the B1b isoform.