A targeting vector containing a FRT-site flanked NEO selection cassette was used to insert loxP sites flanking exon 2. This selection cassette was inserted downstream of exon 2 of the targeted gene, and another loxP site was inserted upstream of exon 2. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination removed exon 2. In situ hybridization confirmed the absence of full-length mRNA in the brain.