The second coding exon was flanked by loxP sites. An frt-flanked neomycin selection cassette was inserted between the loxP site and the 5 end of exon 2. Exon 2 and the neomycin selection cassette was removed by Cre-mediated recombination. Quantitative PCR analysis of homozygous mutant mouse mRNA in cortex and spinal cord confirmed the absence of exon 2 containing mRNA. Western blot analysis of brain extracts of homozygous mice indicated that no detectable encoded protein was present. (J:263474)