The start codon for the B1a isoform of Gabbr1 was converted into a stop codon to prevent translation of the B1a isoform. A putative alternative start site was also mutated and a floxed neomycin cassette was inserted in the introns between exons 2a and 3a. 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 B1a isoform.