Exon 3 (functional domain) was replaced by sequences for LacZ, followed by a translational STOP, and a Neo selection cassette flanked by frt sites. Flp-mediated recombination removed the Neo cassette. The modified locus leads to a fusion protein consisting of exons 1 and 2 and LacZ, while the functional protein is missing. Immunoblot analysis confirmed the complete lack of protein expression in the hippocampus of homozygous mutant mice. (J:308995)